Senbon often serve a medical purpose, being used to strike acupuncture points. They have little killing power, but can be thrown with great accuracy. A user with proper medical knowledge however, can effectively use senbon in battle to incapacitate or even kill their target should they aim for vital spots.
To make these needles more effective, the user can poison the tips of the senbon. An added benefit of throwing these at the enemy is that these are smaller than a kunai or a shuriken, making it harder to see and dodge. As shown by Shikamaru Nara at the Chūnin Exams, ninja can also attach bells to the senbon to trick their opponent. [Source: wikia]