Let's start with the obvious: Most caries use MMS, which severely compresses media sent via text. iMessage gets around this by sending a texts to other iPhones as data and (unlike a lot of Apple things these days) "it just works".
Because Apple runs iMessage, there are a ton of other benefits. You're not limited by a number of carrier and technology constraints. Carriers can't read iMessages and (if you don't enable messages in the cloud) Apple doesn't have access to them, either.
Third, and I can't stress this enough: iMessages can be answered on all your Apple devices. The client on iPad, macOS is seamless. By contrast, the messages for web application from Google is unreliable and glitchy.
Google does not get messaging. It's really unbelievable they have tried countless times and they can't get it right. That's the most Google-thing though now. They have the best core services but any fringe service like Alo, Inbox, Trips gets a half-assed intro which is quickly squandered. It's infuriating.