The beauty of the Internet is that you can read the same document I did, for free!
Yes, fax has a long history of changing its communications protocols and command set, just like dial-up modems did, to improve speed and reliability. This resulted in new ways to negotiate connections (including speed, error correction, CODEC, etc.). CED is just one of those methods, and since fax devices are backwards-compatible, and negotiate to the highest-mutually-supported standards, you can just enable the settings and fuggedaboudit. Perhaps they added this setting at the request of some large service provider, but otherwise, I don't ever see a consumer user needing to change it.