Cosmic Law states that Power (As is our Life-Force, Energies and Abundance) is there for all to use. If your power is unused—lying there, so to speak—then others may use it—without your knowledge! Unless you Claim Your Power and qualify it, by stating as above that it is yours and yours alone to use, etc…
The use of your Power without your knowledge is allowed and there are lots of beings out there that work very hard at keeping you in a state of confusion, so that you won't know that they have and are using your Power! This is one of the important reasons for you to Claim Your Power.