Sometimes the backup chips are brought into play when an incident occurs. Sometimes the backup rack replaces the current rack when it is retired due to wear. This happened with the $1 chips at Hollywood Casino in Aurora, IL. Somehow some of the teal colored backup chips got out and they were trading among collectors in the $40 to $50 range. The backup chips were the same as the regular rack except for the color. The regular rack is purple. Then the casino decided to put the backup rack on the tables and this once rare chip became a current chip in play.

Here is a link to the chip:
http://www.chipguide.com/cgi-bin/search/search.cgi?id=ilauhc