Somehow I have missed the purpose.... I presume its to keep the cutting edge from actually cutting in?
Is this to be used in place of shoes? or with shoes on the blade? Does it end up leaving more snow behind?
Yes, the primary reason is to help keep the plow from digging in. This is of course, helpful when plowing gravel.
My plow has this pipe, and pneumatic casters in place of shoes.
The plow rides primarily on the casters. The cutting edge then can float along on top of the gravel, or since it is a loader frame plow, I can use the bucket tilt mechanism to adjust it so that it rides just above the gravel.
I usually try to pack down the first 3-4 inches of snow, then plowing my drive with this set up is pretty easy.