Back in the heyday of arcade video games, a lot of manufacturers would hide things like this in the code to prevent counterfeiting of the games. PC boards are easy to copy (the schematics were in the service manual), but counterfeiters would just duplicate the ROMs and change a few strings or sprites to give the appearance of a new game. The Easter eggs were triggerable by a certain sequence of moves to show the real authors of the code on-screen.

Here's one example: http://www.digitpress.com/eastereggs/arcaderobotron.htm