Stupid threads are usually posted when the wide majority of the message board the thread was posted on already knows enough (or all of the) information regarding the subject that the message board was created for.
Occationally, new people will show up on said board, asking for information regarding the subject. You can expect 1 or 2 (at most) serious replies actually answering the question. The rest will be stupid posts or flaming. The questions always get answered, but due to the high volume of flames and stupid posts, the real answer always gets lost.