Why was the Wall Built?
Hadrian visited Britain in AD122 and, according to his biographer, writing two centuries later, was 'the first to build a Wall 80 miles long from sea to sea to separate the Romans from the barbarians'.
In that way, a function, probably the chief function, of the Wall, was frontier control, just like modern frontier barriers. here the army enforced the regulations which governed access to the empire. It would appear, from comments about other frontiers, that people could only enter the empire at designated points and travel unarmed and under military escort to specified markets or other places. The Wall would also help to prevent raiding which we know happended on all frontiers. The purpose of the auxilliary units based in the frontier area was entirely different; it was military defence - as well as the protection and policing of the provincials. The placing of forts on the line of the Wall obscured the difference between these two functions. Analysis of the location of the Wall in the landscape indicates that it was not always placed in the best position if defence was the main criterion.
It may also be considered that the Wall formed but one part of a wider system of frontier control. Cavalry are attested at many forts on the line of the Wall. To use these soldiers as frontier guards would be a waste of their skills; it is more likely that they patrolled the area to the north. Certainly later scouting to the north of the Wall is attested as well as treaties between the Romans and their northern neighbours.