Minecraft wiki entry for Combat, associated text: Quick Charge can be useful for reducing the reload time.
Piercing, Piercing IV can be useful for killing enemies in a single row, since a single arrow can penetrate through up to 5 mobs. This enchantment can be used for infinite use of tipped arrows, due to arrows being able to pass through mobs and can be picked up.
Multishot, useful when targeting 3 enemies at once. Also, one should use this enchantment if firework rockets are used as ammunition as it fires 3 rockets that explode, causing more affected area damage. Multishot fires 3 arrows, but only 1 arrow can be picked up which can also be used for infinite use of tipped arrows.
Unbreaking can help counteract the problem where crossbows lose durability three times as fast if using the Multishot enchantment.