Like Shacharit and Minchah, Maariv has its basis in the Temple service. In the Temple, no sacrifices were offered in the evening, but any sacrifical parts that had not been burned during the day could be placed on the Altar and burned at night. Thus, although no sacrificial service was required during the night, it was uncommon for the Altar not to be in use. This explains why Maariv began as a voluntary service; unlike Shacharit and Minchah, which took the place of the required tamid-offerings, the evening service on the Altar was unnecessary if all the parts could be burned during the day.