Originally Posted by FBinNY

While tread is meaningless on dry pavement, and even on wet pavement, it does make a difference in light sand, fine gravel, and light coatings of fine silt mud, or snow. It has no effect rolling straight up, but on turns it makes escape paths for fines, so the tire can reestablish contact faster. Not a help in deep stuff, but some tread can be the difference between going down, and recovering on turns with thin sand cover.

It's why for years (though sadly out of fashion now) racing tires were smooth in the middle, and had some kind of file pattern tread to either side.