He is often used in Thieves Guild missions and provides information for a price. He can also be used as a source for vampires to feed on. If one is in the Thieves Guild, it is recommended not to feed on him as a vampire. If caught, one may be withdrawn from the Thieves Guild.
Puny Ancus is associated with the Thieves Guild quest May the Best Thief Win. However, he is not considered 'Essential' and can be killed before the quest starts.