Another reason to start a thread about an issue is that contacting someone with an issue helps ONLY the person with the problem. Where as if a person starts a thread, others with the exact same issue can find, read and participate in a thread and in turn help them as well.
As for finding people in the different forums with the knowledge, again, badges don't always mean anything. Plenty of people with great knowledge that do not have badges. And by posting in an area with these members WITH badges, if they know the answer, they WILL answer the question too. Its a win win situation.
We all have our areas of expertise and if you KNOW someone knows something about an issue, of course it never hurts to shoot them out a PM. But your chances of getting multiple answers is greater when starting a thread.
Also, lots of us have ways to treat or work with something. BUT...what works for me may NOT work for you for a multitude of reasons. So when others jump in to help someone, many times the right answer for that person becomes apparent. Again, it helps many people instead of one.