why do chickens dust bathe?

Sorry to ask such a stupid question, but I'm just wondering why they do it!   

We made a temporary run on our veggie garden last night thinking they'd enjoy scratting about in new soil with bugs and grassy bits and weeds and veg remains etc, but all they did was pile ontop of one another and have a group dust-bath.  It was funny to watch but made me wonder why they did it!  What pleasure / benefit do they get from rolling in the soil looking like they are about to die on me?

it is to clean themselves (remember dry shampoo powder?) and also to help remove any mites and lice. Some people who have under cover dust baths also mix in mite powder so they can self administer it 

I have never thought of putting the mite powder in the dust bath! great idea!

I put the powder in with the saw dust as they throw it every where, gets where we can't!