Here's the breakdown and why we have them:
* On the public forums, there is a LFG subforum. This is a holdover from before we had the LFG Search feature, but some people still prefer it so we keep it around. If you don't want to use it you can just ignore it. No one will talk about your campaigns on there unless you specifically start a discussion on there about them.
* If you list your campaign in the LFG system (using the toggle on the Campaign Details page), then your LFG listing also gets a forum. This is for people who aren't in your campaign but want to join it to let you know that they want to join, and also to ask questions. You can also do things like post stickies outlining the requirements for the game or special rules for character creation, that sort of thing, so people are aware of them before they join the campaign.
* All campaigns have a private forum on the Campaign Details page (regardless of if they are accepting public player applications or not). This is intended for use by people actually in your campaign to talk about the campaign out of game. Some people use this to post things like schedules and talk OOC. Others use it to do play-by-post between game sessions. But the key here is that only people who are actually in your campaign can read or post in it.
In summary, I think it's obvious why there's a separate LFG forum from the regular campaign forum (obviously you don't want anyone to be able to read or post in your campaign's private forum). Why there are "two" places to discuss LFG, that's a holdover from a long time ago, and there may be a small riot if we tried to get rid of it :-).
Hope that helps!