>>29054414I can't speak for the majority, but I nickname mine. Generally I do it to help differentiate between sets on the same Pokemon without having to check their move sets, as an example I've got a Megazard Y named Charcolt and a Megazard X named Alain.
I do have some with sentimental value though, like my first shiny and some teams from runs I'd started with friends. Really looking forward to Hyper Training in S/M so I can make a couple of those sentimental mons viable competitively.