Something that I think the other answers miss is that often military exercises aren’t about training, at least not entirely. Often they’re constructed to test out possible, sometimes wacky scenarios and evaluate performance. Such as - China is invading Taiwan and our Pacific fleet just simultaneously sank, what do we do? Can we still win and if so how?

This is why sometimes you get those sensational news articles about how “a fleet of speedboats destroyed a US warship!” and the like, not realizing that it was a fleet of literally dozens of boats against a single ship with no other support that wasn’t allowed to use half its weapons and in unfavorable position to maneuver, and also it was a rookie captain vs a guy who literally specializes in rapid motorboat assaults or whatever. That’s the kind of stuff that gets concocted for exercises.