Real answer....
it really should just be called a compression sleeve. I wear one (come at me bros) on my shooting arm because my elbow tends to hurt after I play (tennis/fap elbow). If i haven't lifted for a while and only play for like 30 mins I can usually go without it. But if I plan on playing longer I wear the sleeve. The off arm things is maybe just for protection for fouls.
I get the whole looking like a poser by wearing one, but professional athletes, and brands to an extent, push innovation. I am assuming no one hear wears compression shorts when they play or any sneakers with the latest "tech". Just chuck taylors and bermuda shorts.