Values listed in brackets are for alternate (secondary) fire.
The crossbow is the second-most powerful sniping weapon in the game, with a low cost, high damage, and very high head shot multiplier. It also has nearly unlimited penetration, which makes it potentially one of the most damaging weapons. It is popular for its low cost (meaning it is available early in the game), and the fact that it's useful all throughout the game.
Aiming for the head is important, since it deals quadruple the damage to the head.
Try to line up enemies to take advantage of the penetration. Hitting 4 enemies in the body means the first will take 300 damage, the next will take 225 damage, the next will take 168 damage, and finally the fourth will take 126 damage, equating to over 800 damage for a single bolt.