The barb is mostly meant to aid in staying attached while injecting venom and is meant to still be able to release by twisting

Human skin is more elastic than bee's typical adversaries and the singer becomes stuck when they try to release. It you wait a while and let them try to pull it out carefully without hurting themselves, they might end up going in circles until it works its way free

Other insects mostly. Technically also birds, but birds are too quick and too strong so the fight is usually over before the bee can sting.

Other insects, thats the primary use of their stinger