If it's not obvious, I usually go to some effort to find the true medical necessity. Sure, it's sometimes as banal as "the doctor wanted to limit his liability," but in general this goes to the same reason I'm reading through the patient's chart -- I need to have some understanding of this dude's condition if I'm going to be able to intelligently provide care. If they look like Usain Bolt at his peak, with no history that seems pertinent, then I want to know why they're taking an ambulance, because maybe either someone made a mistake or maybe he has Idiopathic Sudden Head Implosion Disorder and this is something I need to know ahead of time.
That said, risk of fall, can't walk, psych patient, isolation precautions, or monitoring for xyz medical conditions will cover most everyone. Some poor RN is signing whatever I put down, usually without looking at it, so it seems like professional courtesy not to expose them to blatant falsehood.