Hitscan detection is an easy and simple way of calculating used in many shooting games. The hitscan function assumes that the projectile travels in a straight line at infinite speed, an inaccurate but practical simulation of a bullet's velocity. The hitscan method allows for easy programming of weapon functions in games, thus saving some resources, but it does not allow for accurate ballistics simulation. This can be a non-issue in less realistic, high-energy shooters, but is usually undesirable in historic or modern military settings.
Some games opt to use individually modeled bullets, which have their own trajectory and strength. For example, Quake's nailgun uses individually-modeled rounds, while its shotgun uses hitscan. Individually modeled bullets are most often found in games that aim for realistic shooting, such as the ArmA or Crysis franchises.
Uses of Hitscan
Lasers or electricity-based weapons are usually hitscan, because the instantaneous hit makes sense in the context of the weapon's design.
Weapons that fire large and relatively slow-moving rounds, such as rocket launchers and bows, are rarely hitscan.