If your elderly parent sets up a legal power of attorney for you, it gives you a lot of options to work on their behalf while they're still alive. You get to access their bank account, pay their bills, pay taxes and do many other things of this nature.
Imagine that you take this role on for a significant amount of time. You get used doing it, and you don't mind it. Then your parent passes away. Does the power of attorney still matter? Does it still give you any legal abilities?
It does not. A person who has died is not legally able to own property. The estate passes on, perhaps into probate, into a trust or just to the heirs in accordance with the will. No matter how your loved one chose to do it, that property is not theirs after death. This renders your power of attorney useless because there is no property for you to handle.
Who can deal with that property? The job falls to the administrator or the executor of the estate. That may be you, but, if it's not, they are now in control because the estate owns the assets.
For instance, you could go to your parents' bank and try to use your power of attorney to withdraw funds from their account. If the bank knew that they passed away, they would deny your request and freeze the account. They would then wait for the estate executor to contact them about releasing and dividing the funds. You may have had the ability to use that account for months or years, but you don't anymore.
It is very important to understand everything about how a power of attorney works, what rights it gives you and what legal powers you hold.