Well, you likely own some things. They're your property. Legally, they become someone else's property when you die. If you don't have a will, they just go to your next of kin (though determining who that is exactly can sometimes be a legal battle). For most people — especially young people, who don't own much and don't have children — it's often not such a big deal. But the more property you own, or the more things you have, or the more people you want your possessions distributed to, the more complicated it can be to sort everything out. That's why the will exists. It is your legally-binding proclamation of what you want done with your "stuff."
It's helpful to get a lawyer, because wills often have to go through a process called probate, to in fact prove that it does in fact represent your true and valid final wishes. The will needs witnesses to prove that it was you who signed it and that you were of sound mind at the time and not under duress. The language of the will needs to be super clear as to settle any disputes. You need to name an Executor to handle all your affairs in your stead, and that person can't be one of the witnesses, nor should they be one of the beneficiaries, because that could tread some murky legal waters. There's a surprising amount that could go into it. It all depends on how much you own and how important it is to you what becomes of it after you pass. It's especially important for parents as they name guardians for their minor children.