The military and video games have had a long history together, going back to flight simulators before WWII. Of course, there’s been America’s Army, but that was a recruitment tool, a way to gloss over the downsides of the Army, namely the permanency of death and having to follow orders.
The question isn’t “what are these simulators?”, but “what are they not?” Well, they’re not going to teach you how to shoot and they won’t get you buff. What they will do is provide tactics lessons in a classroom environment that can then be put to use on the training grounds. For more info on the what and why, check out this essay by a training games company, and this paper from the National Defense University. They’re not just random commercial games slapped together, but designed from the ground up to meet training demands.
I’ve played FPS games online since the good ol’ days of Doom II. And with some of the squad-based ones simple tactics can make or break your game. Me? I charge in and promptly die. And then proceed to do it again.