Dunno.
The logistics of the campaign were impressive. The remnants of Trajan’s bridge over the Danube are mostly gone now, dynamited in modern times for navigation purposes.
Basically, he used an expensive campaign to take over the gold mining that had been going since the Neolithic, province a rich area, and finance his final success.
His last project was kicking the crap out of the Parthians, taking their capital, and extending the Roman Empire to the Persian Gulf.
His pedophile adopted son Hadrian pulled out (ahem) of Mesopotamia and had to be talked out of withdrawing (well!) from Dacia, the region commemorated on Trajan’s column.
Hadrian’s best known for the Wall by that name, but he also had a catamite that drowned in the Nile. Hadrian was so upset by this loss that he had his butt buddy deified, built shrines to him all over the Empire, and built a non-viable city on the site of the drowning, naming it Antinoos after the dead guy.
The Via Hadrianus was constructed from the city across the rugged desert terrain to the Red Sea, in an attempt to give it some kind of economic life (that failed). When the Via Hadrianus was mapped and surveyed, even the water stops were found mostly intact, and it became apparent that it was likely that the only people who ever used the route were the ones who’d built it for him.
Didn’t he invade Dacia to raid their gold mines to finance the building of Trajan’s Forum, an eternal monument to himself?