The order of deny and permit statements has a major impact on how Cisco IOS interprets IP access lists. For this reason, it's helpful to have a way to easily control the order of lines and insert a line in a given position without having to rewrite the entire list.
One important parameter to manage with any call system is where people are allowed to call from a given extension. For instance, you may not want someone to make long distance calls from a lobby phone. Or you may want to restrict employees from making international calls or directly calling certain extensions.