Why Do Military Planes Fly in Formation?

Wouldn’t flying in formation make them a larger, easier target for a SAM than if they were widely dispersed? The Blue Angels I can understand – that’s a stunt. But over military airports you often see planes in tight formation, and of course, always in war movies.

I asked a pilot friend who flew in Vietnam and he couldn’t give a good answer. He described how you do precision flying, how difficult it is, etc. But not why. When pressed, he said, vaguely, that it was good for training. That's not an answer. I don't get it.