RCS is like sms2.0, it supports better group chats, larger higher quality file transfers, read receipts… That sorta thing

For carriers it is a way to extend the (in my opinion outdated) idea of carrier-based chat system.

For Google it is a way to switch messaging on Android to their proprietary app, at least for some time, as other of their projects falied.

For users it is a way for people using Android certified by Google to normally message people using iPhone and it’s preinstalled chat app.