Primarily, the agent's job is threefold - To be a counselor, negotiator and facilitator. What does that mean? The market sets the general price for a property, but a skilled negotiator with a strategy can net you 3, 4, 5% more. Concessions and sales price can be affected favorably with a good negotiation and knowing what the goals and strategy are. Many agents have taken additional training in the area of negotiation to sharpen that skillset.
Next, a home is the largest investment most people make in a lifetime... why gamble your investment with a part timer? If you are selling your home yourself, aren't you a part-timer? Do you want a part-time doctor, lawyer, accountant? Or do you want someone who actively works in the marketplace on a daily basis? You want someone who gives full time effort to the task and treats real estate like a business so that their business is to get your home SOLD or to find you the right home and CLOSE THE SALE.
Don't forget Realtors are a great resource. The best and busiest realtors know the best and busiest home stagers, inspectors and lenders; you know, that 10% of people who do 90% of the business? It's my opinion the question is not whether to use a Realtor, but which Realtor to use.
Then think about this... You hire a Realtor primarily for what they KNOW about selling homes and secondarily for what they DO to sell homes.
Experience brings you a knowledge base for problem solving and innovative solutions that can be a huge asset when things come up in your transaction. What we do everyday becomes that body of knowledge. And it's for hire.