Once in Silver (on the actual Gameboy, not VC), i got a Taurous and trained it to lvl 40, and then for some reason stuffed it in the PC and never used it again. Still don't know why i bothered training it up though.
idk if it's pointless? but i didn't really understand shiny pokemon so the first shiny i found was a whismur and i thought that the green ears meant it was sick. so i caught it bc i was hoping i could nurse it back to health and make it "normal" again. i thought like if i leveled it up and evolved it that might help lmfao. i mean Technically the green is gone when you evolve it, even if it's shiny. i don't have it anymore, but like. ???? lmfao
Looking back, I once went out of my way to complete the pokedex in diamond. I had never done it before and just felt like I should. Total waste of time. Also as a young, impressionable child, I read that if you healed your Pokemon at the Pokemon center 100 times in a row your Pokemon would become level 100. Yeah it doesn't work like that.
I trained a Nidorino in Blue until it learned Double Kick (in RB it learns this move very late) before evolving it. I did it just for the coverage.
On my first real team in Gold I had a Ditto. It took me a huge amount of time to get it to level 100. I barely used it, however.
In X I went for all the stars on the Trainer Card. When I was done I reset the game.