A durable power of attorney contains language stating that the agent's authority survives your incapacity. Without that language, the authority lapses precisely when it becomes necessary — an ordinary power of attorney ends the moment you can no longer make your own decisions, which is the scenario nearly everyone has in mind when signing one.
That single distinction is why the durable version is the standard recommendation for planning purposes. The whole point of appointing someone is to cover a stroke, an accident, or a dementia diagnosis. A document that quietly expires at the onset of any of those has failed at the only job it was created for.
When it is durable, your agent can keep the mortgage paid, maintain insurance, manage investments, deal with Medicare and Social Security, and handle the ordinary financial maintenance of a life that does not pause because someone is in a hospital. The alternative is a court-supervised conservatorship, which costs money, becomes a matter of public record, and hands decisions to a process rather than a person you chose.
It takes effect on signing, not on incapacity — the agent could technically act tomorrow. Some people find that uncomfortable and prefer a springing version that activates only on a finding of incapacity, though springing documents introduce delay of their own. The tradeoff between immediate authority and delayed activation is worth thinking through rather than defaulting.