Your Will, only begins to apply when you die. Until your death it lies dormant.
An Enduring Power of Attorney on the other hand allows you to appoint an alternate decision maker during your life-time. You can limit the power that you give and specify that it is only to operate when you loose capacity. While you have capacity you can revoke it or change it and the attorney must act in accordance with your directions.
A danger of not having an Enduring Power of Attorney is for example not being able to access funds held in accounts when they may be most needed after a significant accident.
When our lawyers make Wills for clients we almost always make an Enduring Powers of Attorney for them as well.