Good question!
I don't know the OP's reasoning for wanting this account, but I have deleted old Reddit accounts on occasion. In my case, it's because I felt I'd shared too much personal info on my account. Sometimes I felt like I was making a "name" for myself. People had started recognizing me from one forum to the next, and referencing my username in their posts - and that's not something I personally enjoy.
Usually, though, I immediately start up a new account to replace my old one. It feels good to have a fresh start, every now and again.
And while (fortunately, so far, knock on wood) it hasn't been something I've ever felt I had to do, I can easily imagine deleting an account if I ever posted something that I deeply regretted. Sort of a, "SO much shame! I NEVER want to look at this account again!" thing.
Edited to add: Of course, as was brought up above, we can't rule out "rage quitting", either. But, in the OP's defence, she was asking on a different forum. Most rage quits happen right in the same thread, or at least on the same board. She might genuinely have no idea how to delete a profile.