Why do I have to enter my spouses info if filing separate?

[xmasbaby0]

If I am filing a separate return why do I have to list my spouse’s information on my return?

Even if you file separate returns (the worst way to file) you each have to list each other's SSN's and some other information on your own tax return.  The IRS can then cross check to make sure you are not "double dipping" for itemized deductions, dependents, etc.

If you are in a community property state, there is more information that will be needed.

Community property states:  AZ, CA, ID, LA, NV, NM, TX, WA, WI

https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states

[AliciaP1]

When you are Married Filing Separately, you are both required to either use the standard deduction or both itemize deductions and the IRS needs to match the returns up to be sure amounts are not being deducted by both taxpayers (for example mortgage interest on the same home).