Over time marketing campaigns can become stale and lose their effectiveness. Also, some times marketing efforts lose their focus as multiple people take over campaigns or the intended audience develops different interests. An external marketing audit can help identify areas for improvement as well as identify areas that are working well and should have more resources applied to them.
Sometimes the word audit can make people feel uncomfortable, so it is important to understand that the purpose of a marketing audit is to identify areas for improvement as a team, not to identify people that need to be replaced.