Yes, it's been a thing for at least a decade, I imagine it helps with pre-sales online, though it may just be offered as a convenience. It really does help keep the movie from unexpectedly being a lousy experience; if you're stuck in a crappy seat, or your family can't sit together, it's because you picked those seats. As someone else mentioned, it also allows them to bring you dinner and provide that upsell as well.
Even our small independent theater in town has reserved seats, some of which are couches.
The oldest theaters might still be the way they always were, but most have gone to offering direct ticketing (to cut Fandango and friends out) and they need something to offer you to make you buy ahead - being able to select the seat is the carrot.
As European, I can tell that it depends on the kind of cinema, and country.
My experience, being discussed in another thread, is that only big commercial multiplex do it, many small cinemas with more alternative content, usually don't do assigned seats, only ticket reservations.
When I grew up in LA 20+ years ago, seating was way more casual. Now everywhere seems to want assigned seating. I think this is in part because so many theater chains now offer a "premium" dining experience. It's yet another reason I rarely go to theaters anymore, on top of most of the film offerings being crap.
You can find both kinds, in europe especially the cut is very clear, commercial cinemas ALWAYS have assigned sitting. The kind you see at malls and have the Hollywood rotation of marvel shit movies.
Then you have smaller cinemas with indie movies, european movie festival rotation, etc, and many of those in at least 4 or 5 countries in Europe I can confirm do NOT have assigned sitting.
Even our small independent theater in town has reserved seats, some of which are couches.