Every player bans one champion - and so those picks would not be in the random champion pool of everyone else. So if there’s something that’s just way cancer you could at least remove it from your ARAM games for a while. Or if you just get sick of seeing something over and over.
Lux isn’t that bad after riot butchered her with range debuff and dmg dearly/taken modifications. Veigar, Sera or Soraka on the other hand are awful no matter what.