This is a forum post that I made on trainboard.com in response to the question "To someone relatively new to Model Railroading I still don't quite grasp the importance of road numbers. Can someone educate me on this?" Here are my thoughts:
For collectors, particularly in the smaller scales where collecting doesn't require a warehouse for storage, road numbers are important. It is also important for some folks who like to run trains. In NTRAK it seems to be a mixed bag. In our local club there are at least two guys who run long consists (60+ cars) who don't give a hoot that all their cars have the same road number. Heck - nobody can read them anyways without sticking their nose into the layout.
However, there are a ton of 'accidental' collectors. People who buy a lot of items for their consists but want to be able to sell their collection should money get scarce. These folks know that it is hard to sell 100 of the exact same boxcar to a dealer. These folks therefore like to have different road numbers in their consists.
Then you have real collectors for whom these things are very important. When Micro-Trains (an N/Z Scale manufacturer) makes a new car, the first road number produced is always much more desirable and valuable than later runs.
I, myself, fall into the 'accidental' category. I am a runner, and I only buy things that I intend to put on the tracks. However, I like that every car in my 'collection' is unique. Finding out which road numbers I could still attain and at what price was of concern to me. eBay is always limited to what sellers currently have in stock. It is not a good source for learning what is out there in general.
This was the motivation for me to build the N Scale Database that allows you to punch in a road number and find out who made it. Or you can type in a bunch of search words like 'Central Vermont 40 Foot Boxcar' and get a list of who made what with which road numbers.
So yes, to many of us it is just like POKEMON.