The reason why Alexander had a foraging party such a distance behind his main army was mainly due to the fact that although the man provided the men with the basic food rations, i.e.- grain for bread and vegetables, with meat, egg, and fish once a meal once a week, sometimes the soldiers wanted something a bit more variety to their meals.
So they often went out hunting for ingredients, be it various herbs to add to the stew fruits or vegetables, eggs from nearby farms, or even meat, hunted from local wildlife or 'collected' from domestic animals.
With the reason for air quotes in the word collected being there due to the fact that although Alexander made it law that the soldiers should always pay for the food they acquired during such trips, well it was much easier to make the poor peasants 'donate them in the name of their lord' rather than taking out their hard earned gold to pay for them.
A nice shiny armor and a sharp sword tended to do that.
