As we've been migrating one to two sites per week to new domains or under more logical structures (moved) you need to think about all of the old content that existed. Many that create new pages, remove old pages, or do a complete web site overhaul to be more logically structured often forget about the old structure until people start complaining. To the search engine those pages still exist until they learn otherwise. They learn by redirects or by failing several times when looking for your website. Imagine a user doing a search and the first result they choose is from the old site that no longer exists. You've just created a bad experience for your visitor. When this happens many click the back button and go to another site. Can you say lost opportunity? This is where 301 redirects come into play.