. In curling you need friction to slow the "rock" down, cause your not trying to make it go furthest, you are trying to make it accurate. And when you are flying, it is actually the friction of the air moving against the wings that give it lift. The only real thing I can think where friction is undesired is the whole space thing, and skateboarding, and such sports. And also anything that you don't want heat to come in contact with.
