You don't have to give a reason, you can leave the field blank if you want and it makes no difference
If you give a reason, it's just for staff records, yes. Mainly for when we delete posts though, it's nice to see why a post was deleted.
When I delete a post I leave it blank, but it's mostly because I made a mistake, like if I was off topic, or if I said something stupid.
I delete posts if I do get carried away on what I say. Lately, that hasn't happened recently. I also delete posts if I have excessive amounts of bumping on a thread. Usually from event threads that I used to host or businesses I had.
Then say that there. I don't often delete posts, but when I do, I always leave the reason in there, because to me deleting a post seems quite drastic to do.