My wife is purchasing some property with a home on it. The office where she does the household registration book is requesting to see me in person and my passport. We are legally married in Thailand but I have nothing to do with the purchase. Why do they need me?

I'm no expert but i believe you are required to sign to say you have no financial interest or claim in the property.

There were reports from a couple of years ago that a farang husband overseas can sign the document at a Thai embassy or consulate and send it to Thailand and it will be accepted.

We live together in the same city so going there isn't a problem. I'm just curious what the purpose is.

You are legally married and that means that you inherit your wife if she dies. You would then be owner of land (which you can't). I believe the rule in that case you must sell within one year if this were to happen.

Any other reason?

The purpose is as stated above, to officially sign a waver and disclaim any financial interest in the property. You will state that the money for the purchase is/was Sin Suan Tau, your wife's personal assets before marriage.

You are legally married and that means that you inherit your wife if she dies.

And just to clarify that if I may ... "and that means that you inherit your ... wife's Thai property IF she has a legally valid Thai Will in which you are named as the sole beneficiary". If she doesn't have a valid Will and your wife has family then you will only receive a portion of the estate according to Thai law.

The purpose is as stated above, to officially sign a waver and disclaim any financial interest in the property. You will state that the money for the purchase is/was Sin Suan Tau, your wife's personal assets before marriage.

Correct, and this is done at land office where the title is transferred to your wife's name. District office where tabian baan (house book) is done should be ok without seeing you in any case.