It provides a much better group messaging experience than SMS (you can see who’s in a group and add and remove people), delivery/read receipts, better image quality, is encrypted (although that gets somewhat negated by automatic iCloud backups), and is free as long as a data connection is available.
Of course many other messengers offer most of these features too, but for some reason, no alternative has been able to establish itself in the US.
iMessage was heavily integrated into the ios flow when sms was the dominant mobile text messaging system. It's not special, and that's the point. It just worked the way people want texting to work as smart phones gained momentum, and iPhones have so much of the market share that it's way more irritating to use a separate messaging app when you can't change the default integration on ios. I miss the convenience of heavily integrated iMessage comms at least twice per day.
Interesting, I use both Messages and a few third-party messengers, and I wouldn't say that Messages is integrated more deeply with iOS, in the way that e.g. Safari and Mail were for a long time (before you could re-associate http and mailto URLs).
The share sheet just shows my most-frequently-used messengers, as well as direct contact names for my most important contacts, no matter what messenger they're actually on.
The only thing I can't yet do on my third-party messenger is initiate messages from my Apple Watch, but that's presumably due to a lack of a native watch app more than anything.
Of course many other messengers offer most of these features too, but for some reason, no alternative has been able to establish itself in the US.