Sometimes, they join to find out the answer to a problem, or some issue they may have with an Apple product, and they are able to have the matter answered without ever having had to post.
The only reason I joined the forum initially was because I had bought an Apple computer, a MBP, as a belated birthday gift to myself in 2008, and, as what is termed 'a switcher', there was a lot about the Apple world - such as basic commands - and keyboard short-cuts - that I just didn't know.
Hence, I lurked for a while, before joining, and my first post was certainly a question about something to do with my MBP.
This was the first online community I had ever joined, and I couldn't believe that there were places where you could simply post a question, - sometimes even a technical question - and people would try to come up with answers in their own time, out of a sense of decency.