A will is your opportunity to decide what happens to all the property you have worked your entire life to obtain. You can specify that certain things go to certain people. You can also exclude specific persons, preventing them from inheriting anything from your estate. A will can also create trusts to provide for your children, and it allows you to nominate guardians who will raise your children upon your death. It also allows you to choose who will be in charge of wrapping up your affairs (your executor).
Many people have the misconception that without a will, the state will take your property. This is not the case. Illinois law directs that, in the absence of a will, your surviving spouse will receive half of your estate; the other half would be divided among your children. If there is no spouse or children, your estate would be divided among your descendants (parents, brothers, sisters, etc.). Therefore, if you die without a will, people whom you don't want to inherit your belongings may very well end up with a share.