Kicking with the instep or ball are suprisingly different. The instep is a bit faster, gives you about 4 in. more reach, but it also has a larger and softer striking surface. The ball is a bit stronger, gives you about 4 in. more DEPTH in the kick, and the striking surface is harder and smaller. I'm thinking about the roundhouse kick in particular. This makes the instep nice for a fake, or for sparing against an opponent that likes to slip, since the speed and extra distance work for you. Since the st
riking surface is large, it also makes a good stunning blow. The ball kick is better suited for an opponent that blocks. The extra depth you get may get far enough behind the block to do damage. Especially if you can switch from an instep to ball as you get to mislead your opponent. The extra power also may be able to move the block. The larger area of an instep kick just makes it easier to block, and then for your opponent to do damage to the top of your foot (if you're barefoot).
As a side note, changing striking surfaces also works well on an axe-kick. Usually opponents will try to slip, moving just their shoulders and upper-chest out of the way. Extending the ball of your foot (assuming your original striking surface was your heel) as your kick misses their shoulder, you get do some nice damage to the groin as you come down.
