When an infantry digs in it means 2 things First and foremost it becomes a defensive position to fall back upon when the offensive advancements fail and number 2 it gives you a place to have forward command as near the front lines as possible and a place to recuperate for your next offensive incursion. For many the trenches became a pre dug grave when the defensive line failed. Elaborately prepared trenches meant you were there for the long haul with little hope of advancement..