Within the Charmed universe, there is a strong representation of duality: there are the "good ones" and the "bad ones".

The Halliwell sisters are good witches. They constantly have to fight off warlocks and bad entities.

While the girls themselves are not perfect: they display all the common faults of normal human beings, it is constantly emphasized that the powers that they posses can only be used to do good and protect the innocent. When Phoebe uses her power to see in the future to get the winning lottery numbers, the (correct) numbers disappear right off the ticket after the draw, so that she can't claim any winnings.1

• 1. S01E01 - Episode 1 - Something Wicca This Way Comes.