The most likely explanation for why Laura Barton is a superhero is that she is Mockingbird. Why? Mockingbird and Hawkeye are frequently coupled in comic books, after all.
It's also plausible that Laura, who goes by the name Bobbi Morse in the comic books, was once Bobbi Morse until she had to change it when she entered witness protection or went into hiding (or whatever the Bartons' official condition is).
In addition, the watch includes the number 19, which suggests that S.H.I.E.L.D. Agent 19 may have owned it. Bobbi is known as Agent 19 and Mockingbird in the comic books.
Of course, the fact that Mockingbird and Hawkeye, two superhero names with avian themes, ended up together is fairly poetic. But it also gives veteran actress Linda Cardellini a plot that is much more important than what we had been expecting.
However, it is not clear if the MCU is rebooting to give Linda's character a bigger role or if Mockingbird will genuinely have a future in the MCU.
The MCU will finally be providing fans with a very pleasant experience if they can connect Linda Cardellini's Agent 19 to Netflix's Agents of S.H.I.E.L.D. Agent 19, who never uses the moniker Mockingbird in the series.