I have never understood the use of trenches in WWI after the experience of the US Civil War in which mobility was the key to victory. It may have been an accomodation of a stalemated situation, but it never made any sense to me.
Mobility is only useful if you can flank the enemy, surround or get behind them - so they have to divide their focus.
If the enemy has enough men and a long enough trench to wall off the entire area, how can your mobility help you? It’s the poor equivalent of a fortress.
Mobility helps if you can get around the enemy and attack supply lines, key storage areas, capture towns and villages...people. How would one do that if there are trenches blocking all feasible approaches?