What were the functions of these two important areas?
The huge supply fortress of Inchtuthil was never finished. There was sufficient space left near the centre for a large praetorium but the space remained empty when the fort was abandoned c. AD 86/87/
There was, however, a sizeable principia headquarters). The remains of a number of rooms were uncovered but it's not clear which would have been which. The courtyard area was not big enough (I don't think) for a full garrison assembly but it was a good size for a large gathering.