It's also a slight verification in some situations — if I were to get someone's number from a dating app, or chat with someone from Facebook Marketplace and the message was green, I'd be much more hesitant to message them.
It's a negative signal to me if it's green, as they may be using a Google Voice number or any other fake number system. This isn't to say someone couldn't have a burner iPhone, but it's way more effort than simply having a Google Voice number, and I haven't encountered that.