iMessage is Apple's proprietary messaging protocol. Apple Messages is the default (and unchangeable) default SMS app on an iPhone. It uses iMessage rather than SMS when chatting with another Apple Messages user. If you use the app to message someone that isn't using the same app, it falls back to SMS. It's seamless from the iPhone user's side except for the bubble color.

Who cares about the bubble color? People who want to send and receive higher quality pictures and video than SMS/MMS allows and can't or won't convince iphone users to use something other than their default messaging application. The color signifies the capabilities of the chat. Non SMS based or SMS fallback apps (Whatsapp, signal, etc) aren't nearly as big in the US as in other countries. The US also has a much higher percentage of iPhone users than other places. Yes, clique-y children care about the color for clique-y reasons but the capabilities the bubble color indicates are the origin of it. "Oh this guy's on Android, he can only send shitty pictures", "he's on Android - don't put him in the group chat because it breaks it", implying it's Android's fault rather than Apple's exclusionary setup. Again, because it's seamless to them, they don't think they should lift a finger to use anything other than the default messaging app.