What is this button

As the title says I am trying to figure out what this button does it was in the truck when I bought it and I have pushed it and the little light comes on to say it's on but I have looked everywhere around the truck and nothing changes when I push it the truck is a 2004 silverado 2500hd 6.0 hopefully the image comes upView media item 297

The is for the lights on the roof, just above the windshield.