RCS provides properly working group chats and end-to-end encryption with iOS and GMS Android users without having to succeed at convincing people to use Signal instead.

It makes group messaging work a lot better, too. Previously, group chats with a mixture of ios and android users were really buggy leading to iPhone users significantly preferring group chats with all ios, which led to some social exclusion for android users in the US, leading to market dominance for iPhones. Now, with RCS, you can make a functional group chat across platforms and you don't have to convince anybody to install something new.