Often main characters—especially female main characters—exist solely to share their feelings with the reader as things happen to or around them. They don't take the initiative to do anything and they don't significantly affect any other characters' lives. If you erased them from their books, their story worlds would progress just the same without them.
If a main character is not directly responsible for most of the significant events in a story, you don't have a protagonist.
You have a Watson.
How to Recognize a Watson
Take a story. Summarize it through the key plot points only—no extraneous junk. Does the main character's name come up more often than any other? If not, you probably have a Watson on your hands.
If you succinctly summarize any Sherlock Holmes mystery, the name "Watson" rarely comes up at all. If it does, it's only because "Holmes and Watson visit so-and-so" or "Watson accompanies Holmes to such-and-such place." Holmes finds the clients, Holmes does the investigating, Holmes solves the cases and explains them. Watson is a pair of eyes and a hand with a pen, nothing else.
Here are some other famous stories told by Watsons.
The Great Gatsby
How much do we know about Nick Carroway? Very little, because the story isn't about him. He's practically invisible, an eloquent ghost who hovers around the characters who really matter.
To Kill a Mockingbird
Scout Finch is six years old; what could she possibly do? Nothing, really. This novel is about her father, Atticus Finch, and how his job as a defense attorney affects his neighbors and his children.
Pride and Prejudice
Here's a fun question for book club discussion. Who's the protagonist of Pride and Prejudice? I can tell you who it isn't: Elizabeth Bennet.
Elizabeth doesn't initiate a single plot point. Her mother drags her to the ball where the family meets Misters Bingley and Darcy, then cooks up the scheme that forces Elizabeth and Jane to stay at Netherfield. Darcy ropes Elizabeth into dancing with him at the Netherfield ball, then breaks up Bingley and Jane. Charlotte invites Elizabeth to stay at Rosings, where Darcy proposes to her unsuccessfully. Her aunt and uncle take her to tour Pemberley, where Darcy arrives unexpectedly and runs into them. Darcy redeems himself by kissing up to the relatives, bribing Wickham to marry Lydia, and putting Bingley and Jane back together. Eventually he achieves his goal of winning Elizabeth over.
In fact, Elizabeth doesn't do a gosh-darned thing, besides say some witty lines and refuse a couple of proposals. She's 100% reactive. In other words, she's a Watson.
There are pros and cons to writing a Watson. If written accidentally, Watsons can be disastrous. But if written purposefully, Watsons can work out well.
When Watsons Fail Horribly
Protagonists drive the story. So if you're writing about a main character who isn't the protagonist, and you don't know it, you could waste a lot of time and pages writing about things that are not the story.
Imagine if Sir Arthur Conan Doyle believed he was writing The Adventures of John Watson, and he didn't realize that Sherlock Holmes is the real star of the show. The stories might have been all about John Watson looking for a wife, John Watson enjoying tea and crumpets, John Watson reminiscing about his time in the army...and in the meantime, Sherlock Holmes would quietly solve crimes in the background. The Adventures of John Watson would be incredibly boring.
Unfortunately, this is exactly how many modern authors write books for women and girls. The heroine merely tags along while other characters push the plot forward. And since she has nothing to do, she spends 250 pages blathering about sex, cupcakes, shoes, etc. Sex isn't a story. Cupcakes and shoes are also not a story, unless the cupcakes were poisoned and the shoes reveal whodunnit. These things can spice up a novel, but they're not adequate substitutes for its substance.
Here's a brief synopsis of a mystery I read last year.
An alcoholic lawyer is cruelly rejected by her crush, who works in the same firm she does. Then a famous client she's defending embarrasses her during a trial. Her career is over. She flees to the US Virgin Islands, where her parents died in a suspicious "accident" a few years prior.
Ostensibly, she's there to solve the mystery of her parents' deaths. She does this by hiring a creepy private investigator and leaving the case up to him. While she waits, a nightclub singer befriends her and guides her around the island. A friendly ghost saves her from local bullies. A new love interest invites her for romantic strolls.
Then the creepy PI shows up on her doorstep. He declares that he murdered her parents and kidnaps her. She manages to escape from the car before it goes over a cliff, but he isn't so lucky.
This heroine does nothing important that affects the trajectory of the story. She spends the whole book being the victim of the male characters (her crush, her client, the local bullies, the murderous PI) and being rescued by the female ones (the nightclub singer, the ghost). Clues fall into her lap without effort, and the killer obligingly confesses to save her the trouble of interrupting her tropical vacation to catch him.
Yet she hogs the spotlight to catalog every outfit she wears, every tear that falls down her cheek, and every margarita she drinks on the beach. She's the worst kind of Watson.
When Watsons Work
There are legitimate reasons to write a Watson. For example, you might find one useful if your true protagonist is someone unlikeable, or if readers will relate to your Watson much better.
Sherlock Holmes is a terrible person. He's a cold, selfish, condescending show-off. He doesn't feel any sentimental emotions and holds people who do in contempt. He's all brains and no heart.
Watson, on the other hand, is all heart and no brains. He's humble, empathetic, generous, good-natured, and a bit slow on the uptake. He's a man readers can relate to, which makes him a much better choice for the narrator.
The original Watson works because he rarely talks about himself. He obligingly fades into the background and puts the spotlight where it belongs: on his friend Sherlock Holmes. In this way, he becomes the stand-in for the readers themselves.
If The Great Gatsby had been told from the point of view of the titular character, it basically would have been The Sorrows of Young Werther. Gatsby would have spent the whole book whining about Daisy this, Daisy that, and then he'd kill himself. The characters are frustrating enough from an outsider's perspective—they would have been unbearable if we'd been subjected to their inner voices.
To Kill a Mockingbird has the opposite problem. The actual protagonist, Atticus, is too good. He's the perfect father and the perfect gentleman. He's perfectly open-minded, perfectly pious, perfectly clever, and he even handles a gun perfectly. Too much perfection makes for a very dull main character. Readers sympathize more with the rambunctious Scout, who's honest about her feelings and rushes headlong into trouble.
And though Fitzwilliam Darcy is much more of a protagonist than Elizabeth Bennet is, Pride and Prejudice couldn't have been told from his perspective. It's a romantic story aimed predominantly at young women. Young women want to be Elizabeth, dancing at balls and flirting with handsome gentlemen. They don't want to be Darcy, dealing with tedious business and political affairs.
My advice: tread carefully.
I can come up with a handful of good Watsons, but there are many, many bad Watsons out there. Watsons are usually the result of writers not knowing who their protagonist is, not the result of deliberate choice.
You'll also notice that the good examples are quite old. Before the late twentieth century, you could get away with telling stories with main characters that weren't protagonists. But standards today are different. Twenty-first century women aren't supposed to be Elizabeth Bennets, meekly allowing their parents and suitors to control their lives. "Fly on the wall" narrators like Nick Carroway work only in literary novels, and even then they're hard to pull off. Readers of both sexes want to be the detective, not the sidekick. Sweetie watched one episode of Sherlock and was greatly annoyed because John Watson "doesn't do anything."
So unless you have a very strong reason to write a Watson, I would avoid it.