Deep Freeze using FOF charge?

Why does deep freeze use a charge of fingers of frost, if cast on an already frozen target? what benefit does it get in exchange from the charge?
Nothing. It's up to you, the player, to determine when to use it. Good management of FoF charges and gcd's is a skill that differentiates the good from the bad.
The reason it uses the charges is when you have an un-freezeable target. (yeah, I made that word up) The fingers of frost allow you treat your target as if frozen, when the target is a boss it allows the deep freeze to turn into damage and crit chances are calculated as a frozen target. Also works that way for FFB.
If your target is already frozen, you shouldn't DF unless you need to actually CC your target. (stop a cast, etc).

Shatter the nova first, then DF and shatter again.