If you've ever done any kind of development work, you know how it is. Sitting, starring a a CRT for hours and weeks, trying to find the one-character error in the subroutine that makes the title text blink. Naturally, you get a little bit bored. And what helps cure a programmers' boredom? More programming, of course!
Easter eggs are great stress relivers for a coder. Some exaples are Fallout -- by holding down the Shift Key while pushing "Credits", you see a (long) list of quotes by the development staff (that contains some pretty raw language, too). Another example is a software package I won't name that, when you click "Exit" while holding down "shift" and "ctrl", spews out a string of profanity stating just what the developers thought of that project.
I have a feeling that the programmers of that new whiz-bang computer game you've been playing all night like it significantly less then you do.
Also possibly the only way that Microsoft Excel is usefull.