dbshabo,
This is done because of the belief that if some person of disreputable intent could take a complete serial number with some faked documents (i.e. a private Bill of Sale) to the local authorites and say, "I had a firearm stolen some time back, this is it's serial number, this proves it was mine, and guess what.... I've found out who has it now.... Can you help me get it back???"......
Another paranoid reason is to prevent any "alphabet soup" bureaucrat authorities who cruise the Boards from taking down definitive info on who has what in their gun safes.
The chances of these types of things happening are admitedly rather remote, but this is the general idea. ..... and IF (BIG if) it ever did happen, it just might be a major hassle trying to prove it was yours and get people with Gov't paychecks poking their nose into your business before all was said and done. Why take the risk???
Just my thoughts on the issue,
Swampy