Yes. It distinguishes iMessage from SMS/MMS. With iMessage you get read receipts (if turned on), high quality multi-media (instead of just MMS), end-to-end enc

Yes. It distinguishes iMessage from SMS/MMS. With iMessage you get read receipts (if turned on), high quality multi-media (instead of just MMS), end-to-end encryption, the ability to edit or unsend messages for a short time, multi-device messaging, no per-message fee, and probably more. The green bubble is a signal that isn’t available in that conversation. It’s a little like the old padlock in the address bar but far more abstract.