probably for more chances at ideal gear from the vault at the end of every week (lots of high level players have alts with the same class and spec and they play the one with the most luck on getting gear)

e: also seperate raid lockouts

It's sort of rare but not unheard of, I haven't played in years but there are some people who just really enjoy certain classes/specs that want to do raids/m+ more than the daily/weekly lock allows for. It was relatively common in Legion for no lifers, especially people in bleeding edge raiding guilds to make duplicate characters because the legendary drops were randomized and permanently locked to a character, some were dog shit while others were class defining and you could get ones that weren't even applicable for your spec.