Transfer of your assets after death is governed either by your will or by statute if you don't have a will. In order to prevent the State from deciding how your property is disbursed upon your death, you will need a will.
A will can be a very flexible document, disposing of your assets as you wish, setting up trusts to take care of family members after your death and to take advantage of any federal death tax exclusions or credits to which you may be entitled.
A will must be probated, however, resulting in some delay in the disbursement of assets. Also, your will becomes public record when it is probated. If your assets are sufficient to warrant it, and expediency and privacy are concerns, you may want to consider a living trust.