I read a lot, and watch a good amount of TV (and other video). For most things, once is enough. But I find myself repeating favorites as well.
Part of the reason for repetition is that I'm not always looking for the same thing when picking something to read or watch. Much of the time I'm interested in something new, of course. But other times I have other goals. Maybe I'm feeling nostalgic, and wind up looking for something I first encountered years ago. Or perhaps some random event reminded me of a particular book recently. It may just be so long since I last watched a movie that I barely remember it, other than that it was fun.
There's also a comfort factor in revisiting old favorites. When you're watching or reading something for the first time, you're not entirely sure if you'll enjoy it. Maybe that movie will feel like a waste of a couple of hours, or the book that looked so promising on the cover blurb will be dull and lifeless. Can't let that fear prevent trying new things, of course, but occasionally I just want something that I know I'll enjoy.
Much of what I enjoy reading or watching involves long, complex storylines. It's not uncommon for me to go through the first part of a story when it's first released, then have to wait for the rest. If it's been a while since I read the earlier parts, I may revisit those before jumping into the latest release. Mostly this applies to book and movie series, in which releases may be years apart. I've also been known to simply avoid an interesting-looking series until the whole thing is complete, but I don't always want to wait.
Generally, I find that a second or third trip through a particular work will result in learning something new about it. Partially that's just because I can't absorb everything at once - there's always some details that slip by. And in many cases, my perspective changes once I know where the creator is heading with the story. I also find, though, that I see things differently because I've changed. Something I first read years ago in high school or college may look very different to me now, with quite a bit more experience and perspective.
So I'll keep copies of my favorites nearby, whether that be my own physical copies or easy access at the library or on the Internet. Going back to an enjoyable book or video can be just as satisfying as discovering something new.