Because we already know who they are

If it was the Atlanta Falcons FC or Atlanta Braves FC, that would make sense.

Taking out the stupidity of Atlanta United, we already know they are the football/soccer team in town. It's not like football/soccer in Europe where there are football/soccer and basketball and other sports run through a "club" that all have the same name, so we need to distinguish.

It's all fairly dumb.

I don't think they need to have mascots, but we shouldn't have any United teams in MLS (unless a couple of lower league teams join together to form a new MLS team) or FCs (unless they use other team names, which they won't). It's all just trying too hard and is sort of insulting.