A very good thread on this can be found here:
http://www.californiafirefighter.com/archive/index.php/t-5496.html

(HINT: Search for "direct" within the text.)

Quote from the thread, "The reason you go to a direct channel is if you cant hit a repeater or control station NOBODY on the fire ground can hear you at all."

Now, if for some reason I'm wrong with Bohemian Interactive's integration for the Direct Channel and BI defined it only for normal voice communications, it would be of little use, as most players are spread over a large distance and are rarely within normal voice communication of themselves. Although, very likely what is happening, server administrators are integrating patches, improperly redefining Direct Channel, further confusing many players.

According to common radio protocols, I restrict my chatter to the locality I'm in, avoiding overloading of the Side and Global channels.

This needs to be clarified, as there is much chatter on the Internet improperly defining Direct Channel. What I'm seeing are confusing terminology such as "Direct Communication", "Direct speak", etc.

And if somebody is making patches making simple "voice communication" possible, the naming should be completely differentiated from the VON "direct channel", such as "VOICE" all in bold capital letters, in an effort to distinguish the two.