What's the point of decoys? How do I use them properly?

They seem to me like a pretty useless item. Anybody with half a brain can tell pretty quickly that there isn't someone in the middle of the map firing a gun within the first three seconds of a round; they show up red on radar and stay there, unlike real enemies; the only thing I can think of is using it as a fake flash, which seems ineffective, or as a way to scare people in a smoke.

I don't know.