TV Tropes has a great write up on the real reason which is basically artistic licence.
After all real battles in space would look like vessels which appear stationary firing weapons with bolts that are too fast to register slowly damaging each other. A star-fighter that intersected the ion trail of another star-fighter would likely experience impacts that rival small nuclear explosions(because that is basically what they would be) that would tear it apart.
In Universe(IMO made up as an afterthought for the novels):
Each craft has Inertial Dampeners, Stabilizers, and one or more AstroCompass.
The Astrocompass provides the ships computer/astromech with telemetry. There are setting in each ship to keep the trajectory on the plane set by the pilot. In the EU there are several places where pilots of X-wings talk about making adjustments to their plane or where the plane was off causing them to miscalculate... etc. X-wings also have
One of the things the stabilizers do is help to maintain the plane. Most ships have adjustment thrusters to change the ships direction. But the same inertial dampener that keeps the insane g-forces that pilots of the vessels pull from crushing them also prevents inertia from maintaining speed and rotation from the thrust. These forces create the yaw effects.
Fighter craft also have Etheric Rudders which allow them to maneuver much like they would in atmosphere.