Literally everything. iMessage is a real messaging app with all of the bells and whistles. Blue means all of the features you want will work. Green means SMS, which is garbage.
iMessage gets you end to end encryption, read receipts, reactions, larger media sharing, functal group chats, and more.
RCS solves some of the shortcomings of SMS but it’s not a feature that Apple supports.
It’s somewhat pedantic, but the point is really that, in our legal system you are innocent until proven guilty. It’s also the morally correct approach in a lot of ways outside of the legal system.