It’s only for emergencies when you have no mobile service (Apple even say on their website a message can take up to 15 seconds to send). Ig it’s cool, but your phone already connects to satellites for GPS :p
How do they determine if it’s an “emergency”
They don’t. But it’ll be painful to use for normal communication due to the delay (it also only supports text, no media). They specifically mention it for situations where you need to urgently send a message or contact emergency services (still can’t call them, but it does something to let emergency services know you need help).