Just seen your edit. I presumed you was making the whole forum private
I'll explain a bit more anyway. :)
A lot of people find forums via google/yahoo searches when searching for answer to there problem.
Part of the reason for doing it is actually to improve the search engine optimisation of the forum, by cutting out a load of bullshit from the search results. Here for example, Rob posted about his dog, not reefing or even fishkeeping related, and I posted about my car, again, nothing related at all. So that reduces the focus of the forum and is bad for SEO, and equally, nobody from Google would give a damn about that, or find their way here if searching for dobermans, nor would we really want them to.
The rest of the forum is obviously wide open, to be found as much as possible by search engines. The idea of the private version of the board is because the public one is where people are more likely to post reefing stuff, the private one is where people post about what beer they're drinking or that they stood in some dog shit that morning, etc...
The idea being rather than a keyword cloud of this forum looking like;
5. Mazda RX8
it'd be something like
6. bubble magus
There's usually some method hidden somewhere in my madness.
Before becoming a programmer full time, I was a search engine optimisation consultant for various media firms for years a few years ago, so while I'm a bit out of touch now, I'm trying to improve the traffic coming here to help it gain better traction in future.
I'll post some graphs soon, but we're being found for more and more good terms and are getting higher ranked for some too. We're first in the world for "flame scooter blenny", and well ranked for various other terms too.