So much of the original art was destroyed after films were made as the studios only saw it as a tool to do the job. Over the past thirty years animation art has been seen a true art form, one of the few from the 20th Century, values increased for original works especially if they had their original backgrounds.
The studios therefore established art programs to produce limited editions of the original art that appeared in films and these were numbered according to the edition size. 250, 500 or 1000 in various editions. Many studios used the original pencil art to recreate the editions and then hand painted them in the studio as the original art was created.
The backgrounds are usually lithographic and faithful to the one used in the film. These have their own market and values, many, many editions sold out and can only be traded on the secondary market, usually the earlier editions sell for higher prices than the more recent.
Nowadays with so much film made digitally it is the only way to have an edition from many films, there are no original cels used to make the films.