I think a thread is like a conversation. It can go to a lot of different places--often unexpected. While I cannot say I am learning much about cars, I am learning a lot about why AA works for some people. Not the topic of the first few posts, but an interesting one (to me at least).
I spent years in AA and never understood a lot about it until coming to SR. The restrictions on what can be discussed by members is relaxed here, so I am learning a lot that I wish I had known years ago.
And usually I learn the most from long threads that travel through a number of topics.