it was essentially there supply train.

when ancient armies went to more they needed to take more than just the weaponry and equipment they took to battle. they also had to take along supplies for setting up camp, making food, fixing broken equipment that sort of thing. this wouldn't be carried by the soldiers themselves but by their baggage train who would tag along behind the army. It also wasn't uncommon for the spoils of war to be carried along as well as even wives and families.

if the enemy got to all of your supplies for living you would be reduced to a bunch of guys standing around in armour holding pointy things. That works great in battle, but not so much when you have to settle down for the night or when you start to get thirsty

edit: my browser crashed but i'm trying to relocate some sources

A major part of any Roman baggage train would have been foodstuffs. Even a small force would have consumed an incredible amount of supplies, which could not have been carried by individual soldiers or gained by foraging (this is especially true in winter or when operating in Roman territory), thus necessitating a large baggage train. The sheer volume of supplies an army consumed is staggering. It is estimated that Julian’s army of 13,000 at Strasbourg in A.D. 357 (10,000 infantry and 3,000 cavalry) would have required at a minimum 30 tons of grain, 13 tons of fodder, and 30,000 gallons of water each day. This only considers fighting men and horses, the many mules and horses that carried these supplies required fodder of their own. Any army of significant size thus could not operate far from it’s baggage train, and Roman mobility was much constrained by the need to remain close to supplies. That Roman armies, both during the Principate and Late Antiquity, are rarely recorded suffering shortages attest to the efficacy of the Roman supply system.