Not sure I agree...

The real key is being found on google...which when done right, can really account for 80% or more of the traffic (w/ remaining 20% direct, referral from sites, social media)...so really the key question is how to get found, regardless of whether there are realtors (or similar profession there).  That is the real question that should everyone should be asking themselves.

AR can produce good google juice (because it has strong Domain Authority/PR) if you are a paying member and blog well and AR domain is known for real estate.  Wordpress (and other sites that you own can also produce this...and I prioritize my website.  These take more time to develop, but once you've put in the effort, they produce greater results.

In addition, AR is a community which creates many additional benefits (aside from the fun).  1.  Networking - can lead to more referral/biz, 2. Training/learning - has given me many ideas to grow business, 3.  Ability to team up w/ more people for stronger google presence (e.g. I guest blog on other people's sites and many others have done this, I'm included in roundup posts on other people's sites for more links (and google juice) to my WP site.

Hi Val, I think the number of reasons is probably as varied as the people who do it.  Some people are dipping their toe in the water and seeing if they "like" blogging, can produce good blogs and can be committed to it... so its a good place to practice (the not-ready-for-prime-time idea). Some people enjoy the comraderie of the comments and the friendly back-and-forth exchanges and being part of a community of like-minded professionals.  The cumulation of blogs is an unparralled knowledge base that is indexed and accessible; not to mention the ability to ask questions (like you've) just done and have answers returned for "identifiable" sources as you can look up our profiles / background .... instead of an anonymous avatar.  Those are just a few off the top of my head... and that just scratches the surface.

Val, 
This isn't actually a "Realtor Only" site.  Although dominated by agents, it is a site for all who serve the real estate industry in one form or another.  

As Annette Lawrence , Palm Harbor, FL 727-420-4041  said, "you are not wrong"... and I will add the caveat "you are focused too narrowly.  So,  as  Judith Sinnard said, "The reasons are probably as varied as the people who do it".  

When I was an active agent, I blogged here and (more prolifically) on another wite, which all linked to my website.  It's a tangled web we weave. I got dramatic personal and professional benefit from all of it.

Starting here is awesome, because an agent can get their feet wet and build an archive of articles while getting immediate rankings.  After they build the archive, they can keep building on that and (perhaps) shift more toward the agent-to-agent interaction... building a referral, and friendship, network.   

I could go on and on but that's the surface of my thought process.  

Referrals. EVERYONE on this site is OUR CUSTOMER. We gather to stay top-of-mind with our top professional colleagues. Their clients may need help in our area someday. So might they. I spent this weekend with qualified buyers who we originally met . . . here.

As it is shareable to other sites why would it make a difference.

Establishing relationships with other realtors.

I blog on my wordpress sites, Google+ and ActiveRain.... I only care about Google indexing my posts.... why do I need to write on any site other than a real estate platform plus Google+???

This is also a site that is shared with Google+, LinkedIn and other social sites.  Also, blogging here provides referral sources.

We blog here because we have the "flexibility" to come and go as we please without worrying that someone will view our most recent blog of 2 years ago elsewhere and assume we are no longer around.  It's a good place to bounce ideas and opinions off other real estate professionals and get different perspectives that sometimes spark innovation in our own business practices, and then of course - there are the POINTS!!! 

You are not wrong, just misguided.

You assume to know why others are participating on Active Rain, a Realtor only website.

Often, an agent needs to hear a few 'AMENS' from the choir to encourage the writer to continue the direction of their thinking.

Meanwhile, their 'other' sources are delivering adequately and remain secret.  

And what about all the FB,TW, G+, PT options avaiable on AR? Don't they work?

Networking with other agents is good for referral business and there is much to learn from other real estate professionals.

Val Evans actually, ActiveRain reach is not limited to real estate agents only! Others do view these posts.

By blogging here, I learn from others on what is right thing to do, what to blog, how to blog.

Many REALTORS(R) have other sites they blog on.  AR is great for keeping up with trends and ideas.  The blogs also go public to facebook, Goodle , Google+ and of course our own websites.  My blogs that I post on Active Rain are being read by not only Real Estate professionals, but the pubic, as well. 

It takes a while to understand it, but it works. You'll love it!

It definitely cuts down on the "what's real property" type questions!

I'm with Debbie.  I think the real benefit is the boost to Google rankings from here and other sites.

To each their own.

The key here is the SEO that we get on AR, due to a lot of work done behind the scenes to get Google and others to notice us.

Debbie Gartner has this one down...