Usually a blogger will start a forum if there's topics he/she can't cover on the blog because it will be off-topic or maybe they don't have much knowledge about specif topics that their readers keep asking. So a forum will be a good option for them to engage with their readers and answer their questions and get other forum members to answer difficult questions the blog owner doesn't have much knowledge about.
Obviously there are more reasons why most bloggers start forums, maybe even to expand. So I can think of many reasons why bloggers need a forum but there's not a lot of reasons as to why you would want to start a blog for an active forum because you already have people creating content for you.
Your blog will be new, there's a lot of effort that's needed to create the blog, setup wordpress, plugins, tracking tools etc. so I think it's wise to just put that energy on the forum. If you're doing social media, then you can share your most active threads or threads that offer a lot of value.
If you don't have anything that's worth sharing on social media then you create a new thread and just offer value (more like a blog post) then share it on social media instead of creating a new blog. A forum also has sharing buttons so when marketed the right way, those value forum threads can go viral and even get search engine traffic.
My point is, I would never build a forum then build a blog after. Instead I would put all my energy on the forum. But that's just me, I would really like to hear what others think about this. I think it's a good topic
