Looking at the form, you'll see this on the back at the bottom:
Paperwork Reduction Act Notice: [legalese omitted to a point]The information collected on this form is needed to carry out the Customs, Agriculture, and currency laws of the United states. CBP Requires the information on this form to insure that travellers are complying with these laws and to allow us to figure and collect the right amount of duty and tax. Your response is mandatory. [extra legalese omiited]
There you go, it gives a hint as to what they do with these forms. They also are collected for statistical and historical as well as legal enforcement purposes.
Sanosuke!