I feel this thread might spiral into the "Offensive is OP" thread debate...
But is it necessary for my playstyle? Absolutely.
First off, without military ideas, I find I quickly start capping on military points. So, in my first two ideas, I try to grab offensive. It's a great "jack of all trades" idea, with the manpower, force limits, general stats. It is also the "master of one", namely, strategic maneuvering. As others note, it's great for forcing stack wipes, but where it shines for me is it allows me to force battles early in the game. A medium stack (say, 15-25% of the enemies force limit) is separate from the rest of the enemy army by a few provinces? Force march in, kick it around, force march out. Easy victory, and it forces the enemy to stack their armies together, increasing their attrition.
It's my favorite idea group.