I had the digital adapters for some time for TVs I seldom used while I had cable boxes on our main viewing TVs..
Breif history: Years ago I was able to receive quite a few comcast channels on a regular TV tuner without any cable box by hooking up a cable to the atenna inputs on my TV.. Several years ago Comcast went all digitial and stopped including the ability to do this and at that time offered an adapter box at no charge to us. We used those to hook up a couple seldom used TV's in the house. After a couple years they started charging a couple dollars for the adaper boxes which eventually was increased to about $6, or 2/3 the cost of a regular cable box. At that time we got rid of the adaptors and either stopped using the TV's in those rooms or used other sources for those TV's.
The basic difference between an adaptor or a cable box, as far as I could tell, was that the adaptors didn't provide any function beyong changing channels and sending the signal to the TV. For example there was no On Demand, no DVR service options and the channel/program guide had more limited features than on a regular cable box.