I *think* this is good advice for American men planning to get married to a Thai national.
When at the US embassy getting your freedom to marry affidavit, bring your wife's passport with you and get a certified copy.
It's useful/necessary for filing tax US tax forms.
I said *think* because I have not yet filed US tax.
TLDR : Answer Summary
When planning to marry a Thai national, American men should take the wife's passport when obtaining a freedom to marry affidavit at the US embassy. It's important for US tax filing, although some commenters express frustrations regarding US tax laws and dependent claims.