There sure are a lot of movies with "Return" in the title, and they all have one thing in common: they are all sequels in popular film series (well, maybe not so much the tomatoes one). The "Return" in the title is a way of saying, hey guys, remember those movies you liked so much before? Well, they're back! Come spend money to watch them! Usually, using "return" in the title is a classic marketing method to get old fans interested in a new version of a much-loved original.
Still, we keep remembering how much Tolkien hated the "t" word (trilogy). He thought of The Lord of the Rings books as pieces of a whole rather than as a series of separate novels.
The movie version of The Return of the King may have come out two years after The Fellowship of the Ring, making it a real sequel. But all three of the original novels came out at the same time, in 1954. So if you bought Fellowship when it came out, chances are you probably bought Return at the same time.
That's why we think the "return" in The Return of the King has more to do with the plot of the book than with the need to get fans to tune in to the latest entry of this popular series. This title is no marketing scheme. It's a nod to the themes and plot of the book itself.
If you've followed the plot of the first two books, you'll know that there is an empty throne in Gondor just waiting for a certain king's rear end to plop down in it. The Return of the King is finally going to show us the arrival of that king (spoiler alert: it's Aragorn) in his homeland. Hence, the return of the king.
We think it's a pretty snappy title, but Tolkien wasn't happy with it. He thought it gave away too much of what happens in Books 5 and 6. He wrote to his publisher: "On reflection I prefer for Vol. III The War of the Ring, since it gets in the Ring again; and also is more non-committal and gives less hint about the turn of the story" (source, pg. 170).
Sorry, J.R.R., we're going to side with your publisher on this one: The Return of the King sounds a lot more tempting to read than the super-general The War of the Ring, especially since "The War of the Ring" could describe any of the three volumes of The Lord of the Rings. Or all three at once for that matter.