Army Supplies

A Roman legion was a vast body of men who all required food. A soldier's daily grain ration was the equivalent of 1.5 kg (ca. 3 lb 5 oz), which was generally supplemented with other foodstuff.
However, this meant that the total consumption of grain was around 7500 kg a day. Together with up to 500 kg of fodder for the animals this made a substantial amount of food.
In military bases, units were heavily involved in their own supply. Land was set aside for the use of the military to plant crops and graze their animals. These lands were referred to either as prata (meadow), or simply as territorium (territory).
Herds of cattle were also kept, watched over by soldiers called pecuarii (herdsmen). There are reports, particularly in the later empire of large numbers of limitanei (frontier guardsmen) who acted as soldier-farmers, charged with growing the crops for the troops.
Estimates of yield in Roman-style farming vary from 2000 kg to 500 kg per hectare land. These estimates result in land being required in the region between 7.5 km x 7.5 km and 3.5 km x 3.5 km to produce enough grain to feed the men. Add to this the necessity for additional land to grow grain and forage for the animals and one can only conclude that the military bases on the frontiers of the empire were far more than mere fortified headquarters, but large agricultural estates.
It also gives us an impression of the logistical difficulties of bringing up food when the armies were on campaign.
In some areas though grain could simply not be grown on the scale required and had to be imported.
Merchants would fulfil the function of shipping the grain from its point of origin to the army bases. But so too veterans and even some acting soldiers were involved in the trade.
Further food was brought in by hunting expeditions. Archaeologists have unearthed the remains of deer, foxes, even bears in the scrap heaps of military camps.
And yet an army was not supplied with food alone. Wine beer and olive oil had largely to be imported.
But so too, was there a constant need for other materials. Leather, iron and wood for repairs to equipment as well as for heating and cooking.
Clothing, too, would need to be replaced.
And for the maintenance of any army base, stores of building materials would be needed. A regular legionary fortress would be built of something akin to 15000 cubic metres of stone, alongside other materials.

Engineering

Fighting was not the sole purpose of the Roman army.
But so too was it a body capable of great building work. Such expertise in engineering came quite naturally to the Roman army, as it had to build its own camps and forts, if necessary it had to be able to span bridges across rivers and build siege works.
But the army also took part in building projects for civilian use. There was sound reasons for the use of the army in building projects.
For one, if they weren't directly engaged in military campaigns, the legions were largely unproductive, costing the Roman state large sums of money. But the involvement of the soldiers in building works, kept them not only well accustomed to hard physical labour, but also kept them busy ! And it was the widely held belief that busy armies weren't plotting to mutiny, whereas idle armies were. Also the quality of work delivered by the army tended to be better than that of civilian engineers.

Of both military and civilian use was the construction of roads in which the army was heavily involved. But so too were soldiers put to use in the construction of town walls, the digging of shipping canals, the drainage of land, aquaeducts, harbours, even in the cultivation of vineyards. In some rare cases soldiers were even used in mining work.

After construction of public works, the duty of maintenance fell to the local communities. But these communities often made arrangements to pay the army to maintain them, bringing in helpful sources of income to pay the army's huge costs.

Police Duties

Several policing duties fell to the army in the provinces of the empire.
Many such duties played a important role in trade. For it was the army which inspected the weights at market and collected customs payments.
Whenever there was a census (the counting of the people of the empire) it fell to the army as the only institution large enough to handle such a huge operation.
With there being no police force and no customs officials, in the provinces everything regarding law enforcement or border controls rested with the army.
Large numbers of soldiers were detached from their armies and, in small units, provided escort protection to traders, guarded provincial governors, patrolled country roads and towns.
Some troops were even used as prison guards, but this was rare, as it was deemed demeaning work and hence was normally given to slaves.
These activities naturally kept the army in close contact with local people and, one may assume, ensured it some degree of popularity, as it was seen enforcing law and order and protecting trade.