I reread for a lot of reasons, sometimes I'm just in the mood for a particular author/universe and have already read everything in it, so a reread is the only choice. Other times I just want to revisit a book/series and see if I catch new things.
I did a revisit reread recently with Terry Pratchett's Discworld novels. I originally read those mostly by sub-series (all the city guard novels, all the death ones, etc.) but now that I'd read them all, I decided I wanted to see what it was like to read them all in publishing order. So I reread all forty of them over again. I noticed a lot of minor things I hadn't before, with characters having minor cameos and events from past books being referenced in future ones. It was quite fun!
For the other one, I end up rereading some of Harry Harrison's Stainless Steel Rat novels every now and then simply because I want to enjoy that type of humor and I've read pretty much all of them. (The ones I haven't, I've yet to find copies of despite years of searching.)
One thing I've noticed is that if I'm in the mood for a particular style of writing/humor (like the Stainless Steel Rat books), I may not be willing to try something new at that point, simply because it's likely to leave me unsatisfied. Much like how if you're craving a doughnut, but eat a candy bar, it just doesn't satisfy you even though both are sweets. I also go through periods where I want to read nothing but new stuff, even new authors, so it balances out.