It's not marketing in the least... Yes an SP fan will work as a case fan, but it will move less air. An AF fans need to do one thing, move air in and out of the case. An SP fan needs to move air at high enough pressure to generate sufficient turbulence within the fins while moving enough air to prevent the air from heating up too much, which would reduce performance. AF fans don't generate much pressure, because they're not designed to cool things, they're designed to move cool air into the case, and hot air out of the case.