There may come a day when you are not able to manage your finances on your own. You could lose the ability to manage your own finances because of mental incapacity (alzheimer's or dementia) or because of physical incapacity. Or you may simply need someone to be able to sign documents for you or assist in financial transactions because you will not be available to do it yourself.
The people you name are called: "Agent Under Power of Attorney for Financial Matters." They can be given the authority to act for you immediately or only if you are mentally or physically incapable of acting on your own. If you choose to have the Power of Attorney become effective only upon your incapacity, two doctors or the court will have to determine that you are incapacitated before authority is granted.
You can revoke a Power of Attorney at any time, as long as you are competent to do so.