The purpose of the WTA is to protect you from having to use part of your relocation expense reimbursements to pay Federal income tax withholding; it does not cover state taxes, local taxes, Medicare taxes, or Social Security taxes (see § 302-17.22(c) and (d)).
If your agency offers you the choice, the WTA is optional to you. See §§ 302-17.61 through 302-17.69.