Not a Drill

In disaster movies, when the Big Push is about to begin, commanding officers always make this kind of announcement over the intercom:
"Attention attention, all personnel. We are about to engage the enemy. This is not a drill, repeat, this is not a drill!"
Now let's assume that this is what is actually being said in a real emergency. (Actually, if movie scenarios played out in real life, the phrase would be more akin to "OH JESUS CHRIST WE'RE ALL GOING TO DIE", but then again, in real life, you don't often see bug-eyed aliens teaming up with Godzilla and Ming the Merciless to destroy the universe. So there.)
So anyway, bear with me here. If officers in real life also say that "this is not a drill", what's the point of having drills? After all, during a real drill, they won't say this, so everybody knows it's a drill. Right? Or am I overestimating the average IQ of the average GI?