There hasn't been much discussion on how important the concept of changing levels is to fighting in general.
The basic level change is probably familiar to most people from wrestling. If you shoot without changing levels you should prepare to get sprawled flat as a pancake.
The same concept of lowering your level before attacking at a lower level applies very well to every kind of "functional" fighting. When boxing you should level change before firing a body shot if you don't wish to get punched in the head.
Attacking the legs in stickfighting should preclude a level change in order to protect your head. This first came to my knowledge from "Real Contact Stickfighting" with Eric Knauss.
Obviously changing levels is an important concept in defense also. In wrestling a level change usually precludes a sprawl. You could also sprawl by essentially throwing your hips in to the air without a level change. In MMA competition this might not be such a good idea if your opponent fakes a shot (by changing levels!) and you sprawl without first mirroring the level change. The result is you on your hands and knees and the opponent ready to kick you in the head.
In boxing defense a level change is effective, especially when you do not want to take your hands away from your forehead even when defending body shots à la Rodney King's crazy monkey defense system. Again this applies very well in an MMA context.
In stickfighting there are several ways to counter a leg shot. One way is to match the level change and attack at the same time.
Obviously this concept doesn't apply all the time. You can attack to the legs with a kick or throw a shovel hook without much of a level change. In stickfighting one possible counter might be a cross step combined with a downwards strike.
I still think it's a rather cool and important concept. What do you guys think?