I've always wondered what happens to the customs forms that you have to hand in when you enter into the US (not the immigration forms but the blue customs one)?

Do they put all this info into a database or are they required by some privacy law to discard the information after a certain period of time?

Just thought it would be interesting to know, as other places like the EU and Hong Kong don't require these forms at all. Wondered what was the point of them in the USA.