So you can direct how your affairs are to be managed.
This includes distribution of all your assets, irrespective of your net worth. A Will is the only legal way you can tell others how you want your assets to be distributed upon your passing.
Known as Intestacy, your affairs will be dealt via a formula contained in the relevant States Administration and Probate Act. These rules apply to everyone, and do not take account of your individual circumstances or what you may want.