If you die, they'll probably pass to your spouse or children.
Some people make a mess of their beneficiary forms.
Or your company or plan sponsor might make the mess, by severely limiting what you're allowed to do with your money. Name a beneficiary when you open the plan. Some people can't decide who the beneficiary should be, and never get around to naming one. Maybe you're single, with no obvious beneficiary. Maybe you're separating from your spouse and want to leave your options open.