Do you need a will? Most people don't think so, or don't get around to it. While an estate can proceed without a will, i.e. by intestacy, a will can resolve many problems.
For example, if you have minor children, a will can designate guardians in case both parents pass away. Likewise, you may appoint a trustee to handle money and gifts that are to be left to children who aren't yet responsible. You can also put money into trust for children who are adults but not fully responsible. You don't want an 18-year-old to spend their entire inheritance on an expensive car or a trip to Europe when he or she can use the funds to help offset college expenses or for a down-payment on a house.
Another reason to have a will is when you wish to appoint an individual to be the executor - the person in charge of carrying out your wishes and moving the estate through probate. Without a will, your children or spouses who don't get along may all wish to become executor or administrator. This can cause many problems and may require court action - something a will can avoid.
You may also give designate certain personal items or money to go to friends or family who otherwise wouldn't inherit.
For these reasons, it is advisable for everyone to get a will, even if you don't think you have anything to leave behind.