Private forums have a place on every community. The most common and, often, best use of them is for your staff forums, so that you and your moderators can privately discuss issues relevant to the site, without other people being privy to the discussions. That’s the way it should be.
However, some administrators mark all of their forums as private, making it so that you cannot see anything until you have an activated member account. On communities meant for specific groups of people (internal company forums, forums for a family or set of friends to communicate, etc.), that’s fine as those communities are not trying to reach more people. But, if you are trying to bring more people to your forums and your target is growth, your primary forums should be available to the public.
There are various reasons for having your forums public and accessible, but here are three to start you off.
If I don’t know what your site is about, how do I know if I want to join?
Some people decide to make their forums private because they think it’ll make the user more likely to register. i.e. they will want to register, so that they can see what the community is like. But, generally, this strategy will backfire and will result in an opposite effect.
Many potential members want to see what type of discussions your community is already having before they take the plunge by clicking the ‘register’ link. Plenty of members register because they see something that they want to reply to. However, if you make them register just to view those discussions it is quite likely that they will simply leave, never to return.
Search engines can’t index you.
For any community that is really looking to grow, this is reason enough. Search engines represent a good chunk of traffic for most communities and a majority of new traffic for many. They can be your best friend. So, treat them like one by allowing them to look at (and index) your content.
Every new thread on your community is potentially valuable content that someone is looking for. To find it, they’ll probably use a search engine. For example, if you ran a martial arts community and someone posted a topic to discuss the differences between Shotokan Karate and Taekwondo; that thread may very well show up in a search engine result when someone is looking for that information. You’ll get their traffic and, if they like what they see, perhaps their membership, as well. You should be embracing search engines and helping them to spider your content – not shutting them out.
If search engines can’t index your content, you are cutting off a potentially big source of traffic and limiting yourself in a truly unnecessary way.
You look guilty.
You’ve seen them. Music, movie or media forums that just have that certain suspicious look where you know something here just isn’t kosher. And then, just to make sure that they are perceived that way, their forums are private, giving the impression that they are doing something that they don’t want the public (i.e. copyright holders) to easily access.
This will apply to some subject matters more than others. But, if you have a community where people might think you are trading copyrighted media (whether or not you are), hiding your forums makes it look like you might be up to something that’s not too cool – whether it’s true or not.
The bottom line is that if your community is focused on growth and you want people to find you, you have to make sure that they can find your content. If they can’t, finding you gets much harder and caring about you becomes even less likely.