Archaeology is a destructive process (sometimes rather euphemistically known as ‘preservation by record’) and Hadrian’s Wall is obviously a finite resource, so there has to be a very good reason for any component of it to be excavated. Such reasons fall into two categories: the first is for research purposes and the second prior to destruction by development.
Amongst recent research excavations was a series of small trenches used to verify the suspected locations of several milecastles. Why do this? Apart from the obvious increase in knowledge gained by the exercise, managing a large and complex monument like Hadrian’s Wall requires detailed information to enable informed decisions to be made about it and knowing where the milecastles are can be key to this.
Good examples of development-led archaeological excavation include the A1 Western Bypass, which crossed the line of the Wall at West Denton, or the new Northern Development Route west of Carlisle, which did the same near Beaumont.