Why do you need my husbands SSN if we are filing separately?

[cwieman]

To check that he doesn't claim you as a dependent or any relationship that would cause you to be liable for his taxes.  It's for your safety.

[DoninGA]

The TAX CODE requires that you enter the spouse's name and Social Security number on your tax return when filing as Married Filing Separately.

[macuser_22]

When filing separately, if one spouse itemized then both must itemize even if there is nothing to itemize - that spouse cannot take the standard deduction.   The SSN is requires so the spouses tax return can be checked to be sure the law is complied with.