To add on to the examples Claire listed, there's also the case where there's something about the character that you can't handle as a player - plenty of characters in PhanRift come with potential content warnings (for abuse and violence in particular), either due to their canons or from being CRAUs from murdergames. Opt-out posts provide people who are potentially harmed by that kind of content (abuse survivors etc) a way to quietly avoid things that are triggering to them without outing themselves.

Since opt-out is no questions asked, a lot of people are more comfortable with being able to do that.