To specify the person or persons you want in charge of your estate when you die. To specify who will be the guardian of your minor children. Without a will, a court will make that decision for you. To make sure a court can't step in and decide who will be executor of your estate or how your assets will be distributed.
Estate law is a very complicated and specialized branch of law.
Even your family attorney may not be fully versed on the issues that set a top-notch estate lawyer apart from the pack. You want a specialist in the details of estate law, not someone who writes just 20 wills a year. Find an estate attorney by asking people who are most apt to work with them for recommendations. These people include an accountant, a banker or a financial planner. Make sure the person you ask for a recommendation understands your personal needs.