Why do I need my spouses social security number when filing separately after filing together last tax season?

When Filing Married Filing Separately, then you must enter the name and social security number of your spouse.  If you don't enter your spouse's name and SSN, you won't be able to e-file.

For additional information, review the TurboTax articles Should You and Your Spouse File Jointly or Separately? and  Married Filing Separately in community property states

when MFS both must use either the standard deduction or itemized deductions.The spouse's SSN enables the iRS to check the others return for compliance.