Since the arrangement is done in groups of 4 teams per group, just like the champions league, what happens if there's a disqualification of a team that your selected and active players are in the group stage -- say about 4 of them? That's gonna be a loss for the team - especially if the substitute players are regular bench-warmers, right?
You can change one? (didn't read the rules yet) player after each round and whole team after group stage for free. It is a bit more complicated than that but that is the gist of it. You can do more transfers as well but they cost some points so that is not usually a good thing to do.