Power of attorney laws vary by state. There is usually a section of each state's statutes that directly addresses powers of attorney, and there is often a section specifically for durable powers of attorney. Many states require that a durable power of attorney be in writing and contain some specific language about the intent for the power of attorney to continue past the time the person creating the power is capable of managing their financial affairs.
Many states require that there be two witnesses to the creation of the document (and the statutes may say so specifically in the power of attorney chapter or mention another requirement that then requires witnesses). Some state laws may require a notary public to witness the creation and apply their seal to the document.
Formalities such as witnessing and notarization are a way that state legislatures communicate the importance of the documents.They also provide for authentication in the event of a dispute. Even if witnessing and notarizing are not required, having them done would be advantageous.
In creating a power of attorney document, it would be beneficial to have the advice of a legal professional familiar with your state laws in order to ensure that the document is legal.