One of the primary reasons long-time iPhone users refuse to switch to Android is due to iMessage. Android simply lacks an in-house app that offers the same features as iMessage. Google's solution to Android's messaging woes was to through RCS, which is the long-awaited, much-needed replacement for SMS. The main problem with making the service mainstream is that it relies on carriers and phone makers and the rollout has been painfully slow. Google's previous attempt at creating an instant messaging service ended rather poorly so it's about time that they come up with a suitable alternative.
Simply put, RCS merely puts Android's default messaging app on par with third-party alternatives such as WhatsApp or Telegram. It has all the features we take for granted such as read receipts, extended emoji support, the ability to share images and all the good stuff. The only (and arguably the most important feature missing in RCS is end-to-end encryption. Since RCS still technically goes via carriers, it is not possible for Google to encrypt it. Additionally, laws in several countries make it so that local law enforcement can intercept SMS/RCS texts for national security purposes. Google says that they may bake in end-to-end encryption at some time in the future, but that isn't happening anytime soon.
Google is working on a carrier-independent version of RCS
Several carriers are already on board the RCS train, but a lot of them are not. It may take years, maybe even decades for all carriers to support RCS and Google just doesn't have that kind of time. This is about to change as Android users in the UK and France (via The Verge) will be able to opt-in to RCS Chat directly from Google without carrier intervention. Once the feature is implemented, users will see a pop-up in the Android Messages app that will allow them to opt-in. The only catch here is that the recipient has to have an RCS-enabled Android device as well. If you see “Chat” in the Messages text field, then the recipient has RCS enabled. At this point, we don't know how long it'll take for carrier-free RCS to make its way to global markets. SMS is all but dead, but it is unlikely that carriers will let it die without a fight.