Visual effects need to move in a manner consistent with the game world in order to provide a seamless experience for the player. A highly illustrative cartoon-styled game would be best served by exaggerated motions, while a realistic simulation game should strive for realism in its particle motion. Along with timing and scale, the motion of particles conveys the intent of the effect. Powerful, quick motions imply damage; slow and lazy motions imply peaceful designs. Particles can convey organic or mechanical qualities through their motion as well.
Acceleration and deceleration are often overlooked aspects of particle motion that provide realism to particle effects. Linear motion and speed is an immediate immersion breaker for the player, as objects we see in every day life are slowed by friction and other forces.