As a map creator it can be hard to moderate your worlds you have on public. Everywhere there are hackers, crasher and others, who are always harassing other players.
The idea is this :
To make sure that you can add a list of people who are not allowed on the map or moderators who can moderate this map. The people who are on this blacklist will see that they are on it, but they can't join it anymore. I think that this feature could make some maps run much better. And I think that every creator is serious enough not to take advantage of this feature.
This would also relieve the VRChat moderators and give them more time for more important things.