A will is the document a person signs to provide for the orderly disposition of assets after death. Wills do not avoid probate. Wills have no legal authority until the willmaker dies and the original will is delivered to the Probate Court.
Still, everyone with minor children needs a will. It is the only way to appoint the new "parent" of an orphaned child. Special testamentary trust provisions in a will can provide for the management and distribution of assets for your heirs. Additionally, assets can be arranged and coordinated with provisions of the testamentary trusts to avoid death taxes.