Lexis Nexis claims that 55% of American adults do not have a last will and testament or some kind of an estate plan. This means that more than half of American estates have their personal assets pass via intestacy - without a will and pursuant to the statutory scheme of the state where they live.
The main benefit a person loses when they fail to execute a last will and testament is the ability to nominate executors to administer their estate after death. This fiduciary will do everything from collect the decedent's assets, pay the decedent's debts, file the required documents with the probate court, and distribute the assets in accordance with the decedent's final wishes.
Executing a will with a nominated executor and setting out the distribution of your assets is a fail-safe should any assets require probate upon your death. While many assets such as bank accounts, life insurance, and other investments will pass outside of probate via beneficiary/joint ownership designations made with the financial institution, anything left solely in your name (most often cars, houses, and other miscellaneous items) will be part of your estate and likely require probate to address. Should you pass away with any personal debts (medical expenses, funeral costs, etc.), your executor should use the probate assets to pay your creditors.