The scenario that is often painted is the "what if you and your family were in an accident..," which is the easiest to grasp.
Say you have $100, or $100 million. You and your family have this horrible accident. Who would you want this money (or your house/car/coin collection/family farm...whatever you care about) to go to? The rules are straightforward, but they can lead to unwanted consequences if you are not careful.
In the above example, all your immediate family passes: if you parents are alive, it goes equally to them. If one of your parents is gone, if goes half to the alive parent, the rest to any siblings you have. No parents? It goes to your siblings. No parents or siblings? It goes to your grandparents, and if they are gone, then to your grandparent's descendants...that's right, your cousins and even more extended relations.
If any of those situations are not what you want, then you need a will. It gets better. If you don't have any family, at all, when you pass, your money goes to the state. THE STATE. Not your friends or your church or the Boy Scouts, the state gets to decide what you do with your money.
And that's just a will. What a competent estate planning attorney will also provide you in addition to your will are Powers of Attorney, Advanced Healthcare Directives, a Living Will, and other documents to ensure that if the unthinkable happens, you will have a plan in place so that what you want will actually be done, and the people you trust to make decisions on your behalf will have the power to do so.
Feel free to wait to make these documents, but lets hope your crystal ball doesn't steer you wrong.