Here's a more readable version of OP's comment. Sorry if this is breaking any rules, mods.

Okay, so I took a course on this in my undergrad and while that does not make me an expert I have always enjoyed Roman military history. For my answer I make healthy use of Pat Southern's "The Roman Army: A Social and Institutional History" so if you desire further information there is a wealth to be had in there.

First off you have to be aware that throughout the Republic and Empire, Roman military camps evolved as the military situation and threat changed but some of the aspects of them stayed very similar. It is unknown exactly when the practice of creating a marching camp was put in place but was established at some point during the Republic and became de rigueur in the Empire.

Earlier on, marching camps were designed to be temporary, for as short a time as one night to possibly longer stays to allow troops time to fix equipment, rest, and forage. These earlier, Republic era camps were not as regimented as their later counter parts but generally were rectangular shaped with a ditch dug around the perimeter. The soil from the ditch was then thrown inward to create an earthen wall upon which the Roman soldiers possibly deployed a series of wooden caltrop like devices (similar in shape to the anti tank devices seen on the beaches in WWII) made up of wooden "spikes" called pila muralia. There could also be a wooden palisade depending on how much effort the Romans expended on their defences. It is reasoned that a Roman camp of this type would be able to erected within 2 or three hours. Within the camp itself the Romans made a series of straight roads, with one running completely around the inside of the palisade wall wide enough to keep the interior tents safe from missiles coming from without, as well as allowing enough room for the soldiers to form up in the event they had to leave the camp and immediately do battle.

This brings me to another point, the Roman military doctrine around camps and forts themselves. Romans did not view their fortifications as something within which to hide and wait out the enemy. Even in the later periods of the Empire, when the frontiers had been established and the temporary camps of the Republic/Early Empire had been replaced by forts of stone and wood, the Romans viewed their forts more as administrative areas than something like a Medieval castle. Their doctrine was to go out and meet the enemy, not stay holed up inside the fort.

To return to the topic of Roman camps however, as I alluded to before, by the second century AD, the marching camp had begun to evolve from a temporary establishment into a much more permanent structure, and along with the much greater standardization in their construction. Now they were almost exclusively rectangular, with walls of stone, rounded corners and inside they had a series of predefined roads and buildings. In the centre of the camp were the administrative and command buildings such the commander's quarters (the praetorium), the granary (the horrea), and the headquarters (the principia). There were four main streets within the fort, each leading to a gate in the fort wall. The street running across the front of the praetorium was called the via principalis, the street leading to the gate directly in front of the praetorium was the via praetoria, and leading to the rearward gate, the via decumana. In addition there would be barracks for the soldiers as well as workshops and a hospital. These forts could range in size from as small as .6 hectares such as ones found in Germany, or as large as 20 hectares for a legion. There have been a couple camps meant to house two legions at once, like the one found at Vetera. But after a failed coup, Emperor Domitian declared that no two legions may be quartered at the same place.

So to summarize, the Roman military at some point in its Republic past decided it would make marching camps. Over time as the Empire and the borders became more stable and the soldiers less peripatetic, many of the camps were built up into permanent regimental dwellings for the legion that inhabited them.

I hope this helps, again if you want a deeper dive into the Roman military and its practices and history you can pick up Southern's book and give it a read. Thanks.