To visit a place with a local, a person born and grown there : a licensed Tour Guide has a deep knowledge of the culture of the area. He/She can suggest your ideal restaurant as well as show you places which are hidden to the tourists' eyes.
To customize your tour.
To optimize you time : with a local Tour Guide you can skip the lines to enter museums and art sites.
To be safe : your Tour Guide can help you avoid any risk and hidden danger.
To enjoy a relaxing and informative tour: the presence of a local Guide is essential to solve those technical problems wich can irritate you.