It is also due to the creeping barrage, where you would inch your artillery fire forward with your troops behind its shield as the enemy took cover during a barrage and therefore it stopped them from being able to gun down your troops.

"In late 1915 / early 1916, Commonwealth forces began developing a new form of barrage. Beginning close to their own lines, the 'creeping' barrage moved slowly forward, throwing up dirt clouds to obscure the infantry who advanced close behind. The barrage would reach the enemy lines and suppress as normal (by driving men into bunkers or more distant areas) but the attacking infantry would be close enough to storm these lines (once the barrage had crept further forward) before the enemy reacted. That was, at least, the theory."

From http://europeanhistory.about.com/od/worldwar1/p/prcreepingb....