There was a time when buying a home was as convoluted as walking into an real estate office and looking through their "inventory" - that is, the properties that the agents specifically in that office had listed for sale. If you wanted to see a house that wasn't listed by that office, you had to go to the corresponding listing office, and they could tell you the details and sell you the home.
As a buyer, you generally didn't travel this circuit with your "own" agent, you relied on the listing agent to help you. Here's the issue, or at least, the biggest issue: The brokerage is obligated to the sellers to get the most money possible on the sale of their home, to represent their best interest - not yours. The running around has been reduced, but the obligation to the represented parties remains the same!
When a home seller hires an agent to sell the home, the agent has a fiduciary (read: legal) responsibility to his or her clients' best interests. The agent must protect their private information and motivations, market and show the home, etc, but he is also responsible for bringing the highest price the market will bear. All of the agent's research, marketing, and negotiation are with his/her clients needs in mind.
A Real-Estate Agent is A Trained Negotiator
Whether or not you feel well versed in the ins and outs of real estate, having a real-estate agent there to fight your battles for you can pay HUGE dividends in the end.
A Realtor's job is to act as a sort of buffer between the two negotiating parties, diffusing what is almost always an exceptionally emotional process for sellers and buyers alike. In fact, as large-scale transactions go between consumers, this one is downright unique. In these sort of situations, it is your Realtor's job to specifically negotiate the facts, and present your interests in the best possible way, and to eliminate the emotion from the discussion. This is something a principal to the transaction typically can't do on their own.