iMessage is basically proprietary RCS. SMS doesn’t support images, for example. When you send an image via “sms” you’re really probably using “mms” behind the scenes, which has severe limits to quality. If you send an image with imessage, RCS, or any of a variety of custom messaging protocols, you can get the full-quality image.
They also support gimmicks like “reacting” to messages which get overlaid in-line with a heart icon. On SMS it is sent as “MooseBoys loved ‘be right there’”.