There are two kinds of powers of attorney: one for health care, and one for assets. They both allow you to choose the person who will make decisions for you in the event you cannot, either due to mental or physical incapacitation. If you are in a car accident and unconscious for several days or weeks, someone will have to make decisions for you regarding your care and treatment (this hospital or that one, this medication or that one, etc.) as well as your finances. Someone will need access to your bank account to pay your mortgage, utilities, etc. so they do not fall behind.
If you do not choose who this person will be, then the state decides for you. If you are married, then your spouse decides. This is still the case if you have filed for divorce but not yet come to a Judgment. If you have filed for divorce, then it is likely that you do NOT want your estranged spouse making medical and financial decisions for you. In that case, you need to create powers of attorney naming someone else.
Everyone who is 18 or over should have these documents prepared to ensure that their medical wishes are followed and that they have someone who can make both financial and medical decisions for them if they are injured or ill, and unable to do so. Many parents believe that they are the automatic decision-makers for their adult children, but this is not the case. If you have an adult child, too, they should have a power of attorney naming you as agent so that you can make decisions on their behalf, examine medical records, and handle finances should they be injured or in an accident.