A roof water barrier is a self adhering rubberized membrane that protects your roof from leaking due to excessive water conditions. These conditions occur during wind driven rain and even more so during water back from ice dams.
There are several areas of your roof especially susceptible to the water penetration that ice and water barrier will serve to prevent. The entire perimeter of your roof near the eaves is the most obvious during the winter when you can sometimes see the ice dams from the ground. Other areas you may not think about skylights, chimneys, and all the “channels” where roof lines merge.
Although water is highly compliant to the laws of gravity, wind and water back from ice dams cause water to move up, more accurately, under. Roofing nails are covered so downward moving water never reaches them. Since the nails used to secure your roofing shingles also poke holes, this creates ripe conditions for leaks whenever water moves backward.
This is why the self sealing rubberized character of ice and water barrier protects you. It seals around the nails and seals tight between the wood and shingling of your roof.