The purpose of a Declaration of Pre-need Guardian is to choose a preferred person (while you are not incapacitated) that you trust and feel comfortable with to become your guardian one day, in the event that you become incapacitated. The court will then appoint your guardian that you selected, unless the person you selected is found to be unqualified for the position.
By choosing your guardian while you are not incapacitated, you protect your interests (over your person and property) in case someone starts a guardianship proceeding to appoint a guardian over you. You are appointing a person who you believe will make decisions that will benefit you. This is a very useful estate planning tool when planning for the possibility of becoming incapacitated and allows you to determine who will help care for your person and property ahead of time.