The Tempest The Supernatural Quotes Page 1

Though Prospero seems to be performing evil with his magic, it's actually not black magic, as he has been careful to make sure that everyone aboard the ship was safe. His intentions are good, even if his magic doesn't always seem to be.

Prospero's magic does not rely on his ability alone. Instead, nature has a great impact on his ability, and he is humbled and attendant to this fact. He also realizes that, though he is powerful, he is not omnipotent, and he is accordingly respectful of nature. 

Prospero's magic is indeed great, as he was able to undo Sycorax' own spell, which she was not able to undo herself. Besides informing us of Prospero's power, this illustrates that he's not above vanity – he's willing to remind Ariel (and the audience) of just how powerful he is.  This passage also reminds us that Prospero is willing to use his magic for his own personal gain--instead of granting Ariel's freedom after rescuing the sprite from the pine tree, he keeps Ariel as his servant until he can find a way off the island.