You cannot set up a power of attorney once someone has lost the capacity to make decisions and instead have to apply for deputyship at the Court of Protection, which takes longer, costs more and limits how much you can do for them.
A financial power of attorney allows a trusted family member or friend to act on behalf of someone whose mental capacity is in decline or who fears they may not be able to cope in the future. It is not a silver bullet, but it does help. You can find out more by reading Jan's latest blog here.