What was the point of this??( plz srs answers)?

Hi okay my friend got hurt after he was pushed from a ledge about 15 feet so he ended up breaking his pelvis and dislocating hip...we called 911 and i know( i asked earlier cause i didnt know) why the ambulance people put him in a neck brace and backboard and the straps but the thing i didnt get why they put an oxygen mask( the one that covers mouth and nose) on him cause he was breathing fine but why did they do that??? *he's 15 btw???