Hiya Matt,
Some copies are made buy the production studios for use by new animators or older ones that move on to newer productions in the studios as models for the toons so the same model is used by all.
This type of copy is considered production art and can be worth a good chunk of change if it's a rare scene or a copy is all thats left after the original has been destroyed for what ever reason.
These copies also are used to archive a "for use"copy of the scene model to keep the original from being messed up with alot of handling by the animators.
I have a bunch of these types of production copies in my gallery if you would like to see what I'm talking about. Here is a link to them.
http://backlotanimation.rubberslug.com/gallery/master_query.asp?SeriesID=16205
As the others here have said sometimes the only way to get a scene or a shot from a show you like is to buy a copy of it as only one is ever made and this maybe the only way you can get what you want.
Be real careful buying copys online, As most of these are illegal and brake copyrights of the production companys and you can go to jail or pay big fines just for buying or owning them even though you did not make them.
If the ad says you are buying a copy they made. Dont touch it!!!
If the ad says your buying the paper or just paying the shipping and handling and the copy is free you should be ok as this does not infringe on the copyrights of the production companys.
You will see this done in lots of ads for cels that come with a background copy for the cel, The copy most of the time is given for free and this does not brake copyrights of the production companys.
The best thing to do is to buy studio copies from the production company as these are made to be sold to the public.
I hope this helps some.
Roy