Most agents prefer to show homes that have a lockbox - it's easier to fit into their showing schedule, they don't have to deal with another agent present, or a seller still at home. Many sellers are uncomfortable with this - they know or have heard of agents who don't make an appointment, despite the instructions in the MLS, who then arrive at the door, ring the bell or knock on the door, get no response, and let themselves in. More than once an owner has been surprised in the shower, or has been otherwise occupied.
The CBS coded lockbox provides a compromise. The agents lockbox key won't work by itself on a CBS lockbox - they have to call to get a special code to enter into their lockbox in order for it to work on that home. Gee - means they have to make an appointment. That's it - just make an appointment, enter a code, and your key works on this lockbox just like any other.
We used to have the lockboxes programmed at the Board, which was a real pain - now we can do it ourselves, and we can change the security code as often as we want.
Sharon, that's why I don't use combo lockboxes on my listings. I like to have a record/documentation of who entered the property and when. Agents have been known to give buyers the combo... I've witnessed it. Scary!