Yet another Flavian public works project?
You wonder if in 75AD old Vespasian wasn’t thinking back to the good old days with the Second Augusta in Britain and saying; you know, maybe we should beef up the fortifications, pump a little money into the local economy...
The conquest of Britain went pretty easily, as provinces go; the garrison was spread out and fairly small, and that’s considering that Agricola wasn’t allowed to finish up in Scotland. The Romans were caught flatfooted by Boudicca’s rebellion, but committed an extra legion to that, and cleaned it up in no time.
There’s also a Roman site (just one that is known, so far) in Ireland, which was a forward base, and buffer, and trading post, and alas not very studied so far. The Isle of Man was conquered.
Anyway, yeah, having fortified towns made sense, particularly early on.
The construction of the Antonine Wall, and thereafter Hadrian’s Wall, and the still-later reconstruction of the Antonine Wall, shows that it took a while for the Romans to basically give up on plans to finish up the British Isles.