A lot of you will probably remember the magic bar shown in a few screenshots prior to the release of the final version.

Well, magic power was apparently used to transform Link into his wolf form:
"Talk to Midna.You don't have any magic power....You know you can't turn into a.beast without it!"
Not too groundbreaking, although it's nice to know what it would've been used for.
I'm glad they dropped that gameplay mechanic before the release - I didn't miss the magic bar in Twilight Princess for a second and think it would've been very annoying to always have to look for magic power just because the bar is empty.
There's also a beta item called "Magic Potion", the German version calls it - as in previous series installments - "Grünes Elixier" ("Green Potion").