You're filing MFS or single or HoH and need to claim your share of estimates made when you and your (ex-)spouse thought you were going to be MFJ. Because otherwise the IRS will give all the ES credit to the person whose name appears first on the ES voucher.

Ok so it's not a way that we could use to flag to the IRS that on a joint return the tax payments were made under the spouse's SSN to avoid a notice or call when the account truly is paid in full.