The difference between the van driver and the professional tour guide
(or, why we recommend using a licensed tour guide when visiting a museum, or archaeological site):

In order to protect our cultural heritage, the Greek government allows guided tours to be performed only by officially licensed guides to offer their services within archaeological sites, museums and churches throughout Greek territory.              
A professional, licensed tour guide has been trained in a special school for three years so that he/she can accurately present to the visitor all the historical events and bring to life the story of each place, connecting it with life in modern Greece. Choosing to visit an archaeological site with a professional licensed tour guide will enhance your experience and give you the opportunity for an in-depth analysis and presentation of what you see. Also, the history of each site can be related to the history of  other parts of Greece you may visit, so you will eventually have a complete and in-depth experience of your visit to the country. Private guided tours are ideal for families with young children, who are usually fascinated when they visit the home of Alexander the Great, Hercules, Poseidon, etc. getting kids more involved in the tour, which will make it even more interesting for them! Your tour guide can also provide you with accurate information about the various local attractions you can visit, as well as information about the local community and its customs.

The driver will drive you from one site to the other and will be waiting for you at a designated area. He will not accompany you to the archaeological sites and museums because he is not a professional licensed tour guide. Usually drivers know Greek history and mythology and can tell you about the sites on the way there, but their knowledge is rather limited.