Here's three reasons why you might want to post a guest blog instead of dumping your content elsewhere:
1. When you post content on homebrewing forums, you lose your rights to your own content. It's been a while since I have looked at the user agreements of some of the forums but it is very common on any message board to agree to give the owner unlimited rights to the content you post. You might not care if somebody else has the right to use your how-to guide to making beaver anal gland extract (it can be used to make artificial raspberry flavoring) today but in a few years you might feel differently. Especially if that website goes on to use your content for its own profit and removes any attribution to you. You might have a falling out with the message board (or get banned) and want to take your content back. You can't. Someday you may want to restructure that information into an article you would like to publish. You may not be able to have it published if the publisher requires exclusive rights because you already gave out licensing rights to the message board.
Regardless of guest posting, that is an excellent reason to put your content in a place you control and give out links to the information. I'm not anti-message board but I am in favor of protecting my own rights. If you want to guest blog here, I'll put up your post and not keep any other rights to repurpose the content. If you want to take it down someday, I'll take it down at your request. You can do whatever you like with your content.
2. You may want to blog but fear you will not keep it going beyond a handful of posts. It's very well documented that most blogs started do not make it past the two month mark. If you have spent some time searching out homebrewing information you have undoubtedly come across several blogs that lasted a few months and then went silent. What happens is the author starts with a lot of ideas and by the time the author has published all those ideas, it's been about two months and the author runs out of new content (and posting a lot of content to message boards further drains you of content for your blog). Sometimes the author wants to publish more but doesn't have time. Sometimes authors realize after a few posts they don't like writing as much as they thought they did.
If you have a good idea or two and want to see if blogging is for you then guest blogging is a good way to give it a try without committing to developing your own blog platform. Of course, you can always repost your content on your own blog if you decide that is something you want to do. So nothing lost by trying your hand at blogging here first.
3. You want to advertise your own blog/forum/podcast/vlog/social media/brewery. You may already run a blog, twitter campaign, video log, or other platform where you talk about beer, homebrewing, the brewing industry, etc. but you're looking for good opportunities to expand your viewers or subscribers within your niche. Guest blogging is a good way to develop links within the beer and brewing community as well as reach parts of the community you may not organically reach. This blog pulls in between 3,000-4,000 page views per month, so it's a decent amount of exposure.
If you have a new or up-and-coming brewery you are trying to spread the word about then putting up a guest blog would be a good way to spread the word about your brewery as well as give yourself some SEO juice. I am fairly wide open to ideas, even if they are fairly self-promotional as long as there is valuable content. It could be done as a Q&A format if you don't feel up to writing a lengthy post.