A "Self Proving Will" is one that includes the testimony of the witnesses as an attachment to the Will. The Self Proving affidavits are not technically part of the Will and are not required to make the Will valid. It is simply the sworn statement of the witnesses to the will, establishing the legal basis and facts which occurred at the time of the execution which establish the will as valid.
These events and facts are legally required to establish the Will as a valid Texas Will and will need to be established in open court at the time of the probate. Texas permits this testimony to be established via notarized affidavit at the time of the execution so that the presence of the witnesses is not required at the probate hearing. This greatly minimizes the burden on the Executrix to later produce these witnesses who may have moved, died or otherwise are no longer able to be found in court to "Prove-Up the Will".