A good real estate agent has great connections in the real estate world. They have a readily available list of home improvement contractors (heating, roofing, structural, electrical, painting, plumbing etc.), one or two good handymen, a cleaning service, legal referrals and lawn service providers. You should never have to go to the phone book to find help during the marketing process.
A good real estate agent has systems in place to sell homes far more efficiently than a homeowner ever could. Selling or buying a home within the established real estate system is incredibly efficient compared to selling or buying a home outside of the system. Real estate agents have (or should have) a 7 day/week showing service, MLS access, a contracts library, lock boxes, signs and Internet sites.
Obviously, one important reason you hire a real estate agent is because you expect them to know more about selling homes than you do. Selling real estate professionally requires a license and continuing education. Every real estate transaction is a little different, with its own little quirks, glitches and special circumstances. The best way to get in trouble is when you don't know what you don't know!
Your real estate agent will spend a lot of time managing the sale of your home. There is far more going on behind the scenes than holding open houses and attending closings, although due to the above factors (connections, systems and expertise), a good real estate agent will be pretty efficient at their job. The time your agents spends handling the sale of your home will save YOU lots of time... and money!