The short answer is money and control. First of all you want your heirs to be able to avoid Probate. Probate is the court that people have to go to if the deceased dies intestate or without a will or trust. This can be expensive and time consuming, including fees and the cost of a lawyer. That alone makes it worth planning ahead.
Next, If you die without at least a will, if not a will and a trust you will have no control over how your assets are distributed. For example, married couples may assume that their spouse will inherit everything. However, under Missouri law the assets are divided one half to the spouse and one half to the children. If all of the children have the same mother and father then the first $ 20,000 goes to the spouse and the rest is divided in half. Either way it may well not be what you wanted. Another example, step children are not considered heirs if you die intestate. You may have raised the child from a baby, but unless he or she was adopted, they will get nothing if you die without planning. What if you have no spouse or children? Your money will be split among the closest relatives you have. This may not be what you want either. Maybe you want all to go to your favorite cousin and not the one you never liked. Basically, you have no control.
A major issue of control applies to parents of minor children. It is essential that you name a guardian, or the state will do it for you. Furthermore, they may divide up physical guardianship and financial guardianship. The state will probably choose a paid financial guardian, with the money coming out of your estate.
Finally, if you die with no obvious heirs, your entire estate could go to the state instead of to a friend or charity as you may prefer.
The bottom line is it is well worth the effort to plan these things in advance