Unnecessary damage that to me just says someone didn't care enough to have a bookmark method that takes seconds (from grabbing an actual bookmark you can often find for free to slips of paper to taking a photo of the page number etc)

If it's your own book I support your freedom to dog-ear, I just still don't think it's cool to crease pages. It's better than butchering perfectly good books for craft purposes, it's not like I'm calling it out unless I'm asked about it in some way, I just feel sad about the tiny breaks in the pages.

Edit: Consider getting a nice book jacket with an attached bookmark ribbon, they are super cute.