This is a very common and natural way to talk about an emergency. It's an idiom. Maybe some people also say "υπάρχει ανάγκη" but I haven't heard it. "Υπάρχει μια ανάγκη" - There's a need/necessity
Yeah, but this is the medical skills section. If you needed help and shouted "it is necessary", you wouldn't get it.