Hey there! I’ve been regularly waxing for years now and have waxed both powdered and bare. From what I’ve experienced, a thin layer of powder (I usually just use the corn starch baby powder you can find in big bottles at any drug store) helps to lessen the grip wax has on the skin and also seems to help grab the hairs to be pulled. I’m not sure why but I definitely notice a difference. I think maybe since it dries up moisture, it must make it easier for the wax to grip the hair and not lay flat and stick to the skin.
I’d say for sensitive areas like bikini, face, and armpits, powder is a should-have.