>>2135877
From the top of my head:
-You can choose their composition. You can put supply trains and engineers in it wich are essential for long sieges. And in this game stuff like heavy infantry and elephants are king. Plus some kind of units might synergize better with your countries military tradition. Carthage for example can get a shitton of elephants and then a military tradition who makes those elephants extremely cheap.
-You can drill them. This makes them take less damage in battle and provides you with steady military tradition.
-They build roads for as cheap as 10 gold per tile. This are fundamental if you have a large empire plus give you a number of bonuses. Including import routes in your capital.
On top of that you don't get war exhaustion with them unlike with levies, they can get distinction wich makes them better and you can choose their own generals with the better martial stat while the governor of the province can be chosen for his finesse to keep the province loyal.