This is a feature which I think is generally useless these days. The intent was for it to expand certain URLs with a give list of domain names. In general I think this is a bad idea, and the UA should be responsible for this (and it generally does). All sorts of badness can happen if the intermediary is allowed to tack on some set of domains.
That much said, I'd like to get input from the community, if anyone is actually still using this configuration option? It is disabled by default.