What is the deal with drainage mat?

I was reading the cold climate case studies on BuildingScience.com (thanks for the recommendation, good reading). The fairly consistent strategy is 2x6 walls with R19 batts, Tyvek, 1" rigid foam, drainage mat and wood siding. Does the mat make a difference if I am under big overhangs anyway and don't have a lot of direct exposure? Or, is it more a let the wood siding breathe thing and just good practice? I couldn't find it at HD or Lowes? Any recommendations for a good source?

I've never heard of using road mat.! ???????????????????????????????????????????????????????
New one to me.

I have heard of and sold a product called cedar breather, and a few other similar prouducts. These products are really designed to let the siding breath, and dry on the back side, as you have already guessed. I believe the moisture they were trying to eliminate was coming from inside the home. It was pretty important when installing wood shingles on a side wall. Pretty hard to find these days. It has gone out of fashion since homes are so much tighter than they use to be.

If you are thinking you still need / want that breathing room for one reason or another, you can use furring strips. Install them vertically ontop of the foam nail through them into your studs (if you can find them). Leave an air gap at the boottom and a vent at the top of the wall.

This is probably overkill these days. I'm intersted to see why BuildingScience.com suggests this is needed today.

Thanks Murph. Seems to be the consensus of everyone I have talked to as well.