His job was "bridge watch officer." That meant he manned the conn when the captain and first officer were away and filled in as necessary at other posts. It's a real naval position, just not one ST has generally used.
Oh I know but how much can one do with a charcater like that.
A character who can perform any possible function, including commanding the whole ship? I'd imagine you could do quite a lot.
After all, it's not like Trek characters were always defined by their jobs. Data was technically the operations manager, which meant he was responsible for allocating ship resources and scheduling their usage. Pretty dull stuff. But he functioned as the de facto science officer and was also the second officer, putting him in the chain of command. Harry Kim was also an ops manager and de facto science officer. And look at Chekov. Why did Kirk keep bringing his navigator along on landing parties?
Besides, it wasn't Worf's job that led to his popularity, it was Michael Dorn's appeal as an actor. The producers increased the size of his role in response to that popularity. They would've done the same even if he hadn't become security chief. Heck, look at how big a role Colm Meaney's originally nameless "Transporter Chief" grew into because of the actor's popularity. No other transporter chief has ever gone on to become a major character. So it's not about the job, it's about the performer.