Considering that the sketch looks more like a paratrooper platoon leader than someone who would have gone "over the beach," I suspect that the map and such were considered necessary based on what had been learned from the drops in Sicily.
ETA: From reading the Slate post, it seems odd that this would have been based on someone with the 29th. The whole rig is very 82nd/101st. There was a hope, I think, that the breakthrough would be quick, but I don't think there was any realistic possibility of changing the landing locations.
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