There are several reasons why someone would want to cancel their Power of Attorney:
The Power of Attorney is not needed anymore: Perhaps the principal is now able to act for themselves, or the Power of Attorney was created for a specific purpose that's now finished. In both circumstances, the Power of Attorney is no longer needed and should be cancelled to stop the attorney-in-fact from making decisions in the future.
The attorney-in-fact is not fit to act for the principal: For instance, if it is discovered that the attorney-in-fact is untrustworthy or careless, the principal may decide to revoke the Power of Attorney and appoint someone to act on their behalf.
The attorney-in-fact can no longer act: For example, if the attorney-in-fact moves to another country, it may not be practical or possible for them to keep acting for the principal.
The principal wishes to appoint someone else as their attorney-in-fact: For instance, if your attorney-in-fact passes away or you've named your spouse as your attorney-in-fact and you divorce, you will need to appoint someone new to handle your affairs. That said, in some states, such as Alabama, a Power of Attorney is automatically revoked if your spouse is your attorney-in-fact and you divorce.