If you wait until it become necessary you may not be able to set it up. The person signing it has to be completely compos mentis (and conscious too, of course - if you were suddenly struck down and lying in a coma it would be too late) If I were about to undergo major surgery, for instance, I would have the poa completed as well as a will (hope that doesn't sound morbid)
DH and I set up power of attorney for each other, with the children is the next resort, when we were both quite fit. We did financial poa and a medical one too, in case we were not up to making decisions for ourselves on medical matters. The forms were witnessed and registered and stored beside our wills to be brought out if needed. So far they have not been (DH was lucid almost up to his death)
It is much more difficult to get authority to deal with somone's affairs if they are unable to give their consent - that is one reason for the crumbling houses you see which cannot be sold because their owners are still alive, but not capable of the legalities of a sale.