A person who dies without a will is said to have died intestate. If you die intestate, your estate will be distributed according to state law, not according to your wishes. With very few exceptions, only family members will inherit your estate if you die intestate. If you want to leave money to a close friend, church, or charity, for example, you need to have a will.
And maybe more importantly, if you die intestate, a judge will decide who raises your minor children. Although a judge may appoint the same person as guardian that you would have, the only way to be absolutely certain that you can choose the person you want to raise your minor children is to appoint a guardian through a will.