It just works. As soon as I log into my Apple account on any Apple device I can access all of my messages, and seamlessly carry on a conversation on whatever device is at hand. It works with SMS as well, and I just don't have to think about it.
I know that WhatsApp is similar, but their desktop app, especially authentication, kinda sucks (I need my phone to login, and it seemingly logs me out weekly). Whatsapp is also FILLED with weird porn spam, and scammers.