"Back To My Mac" was introduced as a way to hand-off certain applications (such as Mail or Safari) between Macs. Unfortunately, if you enabled it in iCloud and then later wanted to disable it, you could not get rid of it just within System Preferences: iCloud. Even rebooting both Macs in case the setting was persistent was not enough. I called Apple Care and they eventually found that once Back To My Mac was enabled, it added a Handoff option to System Preferences: General that also had to be unchecked. That option would not be there unless Back To My Mac had been previously checked in iCloud.