What's the difference between Visibility and Preselection of visibility?

Visibility: Here you select the options which you want to provide your users when adding a new event. Based on your selection of activated options the users then can decide, for whom they want to make the event visibile.

Preselection of visibility: Here you make a preselection of the active options of the visibility, which the user, who adds an event sees, e.g. by default "Only my contacts". Of course the user can select another option.