+1 for "It looks cool on-screen" and +1 for "We are used to seeing aircraft banking to turn". But even if it's because the more powerful manoeuvring thrusters point upwards, it is still all nonsense.
The reality of turning is space is best illustrated by the old video game "Asteroids", where the manoeuvring thrusters only rotate the ship about its centre, leaving it pointing a different way but still moving in the original direction due to Newton's First Law. Firing the main engines doesn't immediately make the ship move off in the direction it's pointing, because the original velocity doesn't just go away. Instead, the ship goes in a direction that is a compromise between where it was going before and where it's pointing now. The longer the main engines are firing, the closer the direction it moves gets to the direction it is pointing.
So if I'm moving West and turn the ship to point North then fire my main engines, I start to change direction towards West-North-West, then a bit later I will be moving North-West, then later still I will be moving North-North-West. I will never be moving totally northwards unless I turn my ship a bit towards the East and use my main engines to eliminate my westward drift.
So most space dogfighting would be sideways, like rally cars on a muddy track.
If you want to turn your ship in a semi-circle, you have to keep its nose pointing at the centre of the circle. If you are chasing another ship around the semi-circle and trying to fire at it, having nose guns pointing at the centre of the circle is no use at all! So X-wing and Tie fighter guns would rarely be pointing at their targets. Only ships with movable guns (like the Millenium Falcon) would have much chance of scoring any hits.
So the Millenium Falcon banks to turn simply to keep the dogfights looking like the kind of dogfights we are used to.