If you don't have one, a court decides who gets your assets.
A will is a device that lets you tell the world who you want to get your assets. Die without one, and the state decides who gets what, without regard to your wishes or your heirs' needs.
So-called intestacy laws vary considerably from state to state. In general, though, if you die and leave a spouse and kids, your assets will be split between your surviving mate and children. If you're single with no children, then the state is likely to decide who among your blood relatives will inherit your estate.
Making a will is especially important for people with young children, because wills are the best way to transfer guardianship of minors.
You may amend your will at any time. In fact, it's a good idea to review it periodically and especially when your marital status changes. At the same time, review your beneficiary designations for your 401(k), IRA, pension, and life insurance policy since those accounts will be transferred automatically to your named beneficiaries when you die.
A will is also useful if you have a trust. A trust is a legal mechanism that lets you put conditions on how your assets are distributed after you die and it often lets you minimize gift and estate taxes. But you still need a will since most trusts deal only with specific assets such as life insurance or a piece of property, but not the sum total of your holdings.
Even if you have what's known as a revocable living trust in which you can put the bulk of your assets, you still need what's known as a pour-over will. In addition to letting you name a guardian for your children, a pour-over will ensures that all the assets you intended to put into trust are put there even if you fail to retitle some of them before your death.
Any assets that are not retitled in the name of the trust are considered subject to probate. As a result, if you haven't specified in a will who should get those assets, a court may decide to distribute them to heirs whom you may not have chosen.