How does your question make you sound like an ass? Its perfectly valid...
Anyways, I think people post their serial#'s just as a way to show off their pride and enthusiasm for owning a piece of SNK history.
Usually they'll put something along the lines of "Proud owner of" or "Proud NEO-owner since", etc...
Someone once said it was a neat idea that would allow you to see who else had a similar numbered system as you.
On the technical side, like deffty just said, the serials determine the video output of the different boards SNK manufactured over time. Supposedly the older systems' boards have more parts and output a better RGB signal than later ones. Also, they're said to be more suitable for video modifications.
Don't know much else about the numbers beyond that...