In brief
A will deals with your wishes after death. An LPA appoints someone to make specified decisions during your lifetime. They serve different purposes, so having one does not replace consideration of the other. LPA rules described here apply to England and Wales.
Two documents, two different moments
A will records how you want your estate to be dealt with after death and names executors. A lasting power of attorney is an arrangement for decisions during your lifetime. Making one does not replace the need to consider the other.
Who can make decisions?
In England and Wales, separate LPAs cover property and financial affairs, and health and welfare. They must be made while you have the required capacity and registered before use. Other UK jurisdictions have different arrangements.
A practical starting point
Think about the people you trust, the responsibilities involved and any existing documents. Keep a list of questions to discuss with the relevant professional. Marriage, a new child, moving home or a change in health may be a useful prompt to revisit your plans.
Source: GOV.UK: Lasting powers of attorney