A reserve force in the rear is a permission slip, and permission slips lower the effort of everyone in front.
Shrinking a force worsens the arithmetic and improves the state, which is a trade almost nobody makes on purpose. Soldiers who know thousands are camped behind them fight like people who can afford to lose; soldiers who know no one is coming fight like people who cannot. The alternatives walk home in daylight, and what remains is the absence of anywhere else to put the night.
Rations and trumpets, enough for one meal and one noise, is provision shaped to a single outcome, and provision shaped that way tells the body what is coming. The cost is that failure stops being survivable, and once that is true the valley below stops being a risk to weigh and becomes the only direction available.