Have anyone noticed that the reason why some persons post go off the topic is because they don't read the entire op writing before they reply to a thread, some persons respond to a thread based on the caution of the topic and that is why we meet up different opinions or suggestions that is not in lines what the OP points of view.
I would like to remind you that there can be posts which aren't related to OP but aren't off topic either. Sometime when you find someone's reply that contain valuable information or contain irrelevant information then it's not of topic to quote that reply and share your opinions about it.
In some cases it could lead to discussions which could be off topic ones but in most cases all the discussions in a thread a relevant ones. Some of those are direct responses to OP's thread while the rest of those are responses to other posts of the thread.