I think different people have different ideas about what a thread is and how it should be managed and and how peripheral it may become. I think some just want to start a conversation for the mere sake of starting dialog and don't care too much where it goes as long as everyone gets along. These should be in fellowship and such forums. Others start one for trying to teach or ask about a very specific question and do not want to "waste time" getting to the answer, conclusion, session's end or such w/all kinds of tangent and irrelevant dialog. The person starting the thread cannot be expected to know anything past what topic they introduce and it protects their ignorance of unknown or inexperienced knowledge. There are many reasons. It is important, I think, to contemplate possible tangents ahead of time and dismiss them in the OP. One thing about starting a thread is that you must maintain vigilance in keeping current with its progress so that you can put a stop to undesirable tangents. Another problem is that when one achieves their goal with the thread they abandon it to wherever it may go instead of having staff put an end to it, which is very easy and usually quickly done.