Rubber mat behind rear seats?

What's with the big rubber mat behind the rear seats? On my double cab there's a big rubber mat that sits behind the seats and fastens to the rear of the cab. Is this for some sort of sound proofing?

I've noticed it's a major source of lingering cigarette smoke smell in my truck after I got the smell out of everything else. I can't find it on parts websites and am considering throwing it out and just slapping in some dynamat or something.

 

Just a basic sound deadener.