A self-proving will has sworn statements called affidavits attached to the will. These affidavits are from the witnesses. They state that, according to the witnesses, the will was signed by the testator and that the testator had the mental capacity to execute a will. A notary public then certifies that the affidavits are signed by the witnesses.
It is a good idea to have these affidavits attached to your will. When a will is probated, it must be proved. To prove a will, a court must determine that the will meets the requirements set by law. The will must be proven before the instructions in the will can be followed. One requirement set by law is that a testator must have the mental capacity to execute a will. Another requirement is that the will must be signed by the testator in front of two witnesses. The witnesses would then be required to testify at probate that the testator did have the mental capacity to execute the will.
However, if you have a self-proving will, the witnesses are not needed for probate. The affidavits substitute for having actual testimony from the witnesses. A self-proving will helps avoid the difficulty of tracking down witnesses to a will.
If the will is ever contested, however, the affidavits cannot be used. That is to say, if anyone objects and says that the testator did not have the mental capacity to execute the will, then the affidavits are not adequate. In such situations, the witnesses are required to testify.
For more information, see: W. Va. Code § 41-5-15 (2015); Christopher Vaeth, Annotation, Proper Execution of Self-Proving Affidavit as Validating or Otherwise Curing Defect in Execution of Will Itself, 1 A.L.R.5th 965 (1992); Thomas M. Fleming, Annotation, Sufficiency of Evidence to Support Grant of Summary Judgment in Will Probate or Contest Proceedings, 53 A.L.R.4th 561 (1987); Rush v. Brannon, 82 W. Va. 58, 95 S.E. 521 (1918).