If it was possible that anything could be 100% pure, drawing gas out of the ground could be a piece of cake. Gas, unadulterated by any other substance could travel easily through the pipes without damaging a bit of the structure designed for its capture. Unfortunately, this isn't how the world works.
All three states of matter are represented in the process of extracting natural gas from the earth - solids (sand) and liquid (water) come along for the ride. This wrecks havoc on the pipeline, especially at T-junctions and choke valves. Sand banging against flow obstructions causes corrosion which ultimately leads to economic and production losses.