Some city tours can take you into locations that aren't open to the public. For example, if you visit the United Kingdom or Ireland, you will be able to visit the various castles and maybe even get inside to meet some pretty great people and see the grounds.
Are you looking for a private getaway? Often, city tours will allow you to make reservations for your group so that your group is the only group on the tour. This is great for a larger family or a group of friends that would like a more intimate outing with an educated tour guide.