A power of appointment is special planning tool that not many people are aware of. What if you didn't know exactly how you wanted your assets distributed in the future at your death, but you did trust someone else, such as your spouse or child, to make those decisions for you? A power of appointment is a special authority that you can pass to someone to make distribution decisions for your assets in the future. For example, Husband's Will keeps his assets in trust for the life of Wife, and then it gives Wife the authority, at her death, to distribute the assets out as she chooses. Thus, if Son is in a bad financial situation or marital situation and it is not wise to give the money to Son now, the surviving spouse (Wife) could say that Son's share will stay in trust and not be given out to him. Or conversely, rather than keeping it in trust, it could be paid immediately to Son if his affairs are in order. If flexibility for future decisions is sought, this can be an answer.
This decision deferral, called a "Power of Appointment", is an effective estate planning tool. You can also limit how much discretion the appointee (person with the power) has so that they can't distribute it to someone that you have no intention of providing funds.