(By Chip MacGregor, President, MacGregor Literary, Inc.)
Most authors don't know all the things I listed in the previous post and need a specialist to assist them. A good agent brings access through his or her relationships in the industry.
Long ago publishers realized the value of agents, and generally they won't look at unsolicited manuscripts. They insist that all proposals come through a legitimate agent. Think about selling your home: you can do it on your own (my wife and I have sold houses "By Owner"), but it's not easy. You have to educate yourself to make sure everything is legal and that the deal is done appropriately and fairly.
If you own an expensive home, it's tough to sell it yourself. Buyers want the professionalism that comes from having the assistance of a good realtor overseeing the sale. Similarly, when you sign a book contract, you're agreeing to a series of legal clauses that will govern your book for as long as it's in print.
Having somebody help you through the process is always helpful and often necessary. Having someone assist you with the long-term view of a writing career is usually deemed important by most career authors.