A few things you might not be considering: The creeping barrage would hide the advancing soldiers behind dust clouds. The closer to the troops the better (hard

A few things you might not be considering: The creeping barrage would hide the advancing soldiers behind dust clouds. The closer to the troops the better (harder to get sight lines on them unless you were literally on top of them). Enemy positions were not always fully known and mapped out, so firing "just" at the opposing trench would be hard to plan. There could be advance defensive positions placed further forward as well. There were multiple layers of trenches which an advance could be expected to push through. The shape and pace of the creeping barrage was thought to help coordinate the pace of the advance. There were drawbacks, as of course many shells would hit empty land and so be wasteful of artillery, and creeping barrages did fall out of favour. Although not simply for this reason: later developments made it easier for infantry to call artillery down on specific positions as needed, and also new infiltrations tactics made coordinating with creeping barrages impossible.