Why do manufacturers supply a fired case with a new gun?

Nothing to do with bullseye shooting, but I bought a ruger gp 100 as a sport/fun gun and there was a fired case from the revolver included in a small envelope. I searched on the internet and never really got a sensible answer. If it is to be able to link a firearm to a crime, why do I need the spent case? The spent case I got had a severly cratered primer.

I'm told it has more to do with the fired bullet that you didn't get back.  Apparently the feds keep a database regarding the fingerprint of the fired bullet.  I don't know why they put the spent case with the gun instead of recycling the brass.

Some States collect the brass when the gun is registered. Most don't. They duke it out pretty thoroughly on this chat site.. Lots of opinions

http://www.thehighroad.org/archive/index.php/t-438703.html

What Otiso is talking about is NIBIN.  You can read about it in Wikipedia/Google.  Basically, when a cartridge is fired, the breech face of the slide leaves it's "fingerprint" on the back of the casing.  Supposedly, no two are alike.  The casings collected are placed in a data base to see if other casings recovered will match the one's on file.  When ever a gun is seized, it is fired and the case collected is anlyzed and placed in to NIBIN.      

As far as why an empty case comes with a firearm, I don't know.

from ruger firearms faqs page

Q-Why does Ruger ship a fired cartridge case with every pistol and revolver?


Certain states require that all newly manufactured pistols and revolvers be accompanied by a cartridge case that has been test fired from that gun at the factory. The case must be placed in a sealed container bearing certain information concerning this test cartridge. At the time of retail sale in those jurisdictions with the requirement, firearms dealers must forward the test cartridge to a designated destination such as their State Police Laboratory. As independent Ruger Distributors have nationwide markets without territorial restrictions, we have decided to include a fired cartridge case with all new Ruger pistols and revolvers to minimize the possibility of inadvertent non-compliance with these laws. Retail customers located in states that do not require a fired cartridge case may be assured that the fired cartridge case they receive with the firearm at the time of retail purchase is proof that your new Ruger firearm has undergone our normal test firing procedures during manufacture.

Ok I got it. 

I thought they were bragging about the strength of the firearm when I saw the cratered primer....and boy was it cratered.  
I guess what they really wanted was to make sure that every machine mark unique to that revolver would show up in those jurisdictions that required a footprint and the only way to do that was to lay the powder to it.

I think even New York got rid of this requirement, so it might just be Maryland State Police that asks for the fired case. Apparently, they collect them in blue plastic barrels and never log them in because the program got defunded some time ago. They still demand the FFL send it in though.