From what I've gathered, the military tends to send civilians outside the Quarantine Zones on regular work details every few months. The military probably provides security, while the civilians might be used for scavenging for supplies. There are likely some civilians that have better/unique skills (mechanical, lockpicking for getting into safes, etc.) than their military counterparts.
Indeed, plus from a story-telling perspective, it helps assert the atmosphere that the military are less protectors, as initially intended, within the QZs but actually authoritarian dictators enforcing their will on the lowly residents since the military controls the rations, guns and fuel all the residents come to the city to have definitive access to. Therefore, the military use their complete control to make the people work for their 'reward' first. Earned protection and food, with the 'earned' part costing the people their freedom.
Doubt they're cleaning anything up. I imagine that the dudes in the yellow jumpsuits are doing stuff like taking apart old cars for scrap metal, doing menial labor like carrying military supplies, maybe scavenging.