What is"Program Switch" on iMac side panel?

I've had this iMac for a long time and I thought I knew all its features. I came across "Program Switch" in a graphic of the side panel posted by Duane a few weeks ago in the following thread:

http://discussions.apple.com/thread.jspa?messageID=7678269&#7678269

I never knew it existed. I've looked everywhere: in Mac Help, Googled, searched in this forum, looked in the original instruction booklet (which shows it but doesn't name or explain it), but have come up with absolutely nothing to explain what "Program Switch" is.

I suppose it can't be that important since I've been using this computer for the past seven years without knowing, but does anyone know what "Program Switch" is? Thanks.