I'm just wondering what is the point of reposting everyone'should post before replying to it. I don't post enough to understand why ?

should... should not be in there. lol

In a slow thread with only a couple of participants, it doesn't contribute much. But in a busy thread, it helps make clear exactly who and what you're responding to.

Are you asking about repeating the original question or making a more specific reference to some of those monstrous "quote" chains that pop up from time to time?

As Vorpal Sword mentions repeating the question or just quoting it can really help keep things clear when trying to figure out what your post is actually talking about. If/when threads make a second page it really can be helpful to be able to see the actual question being asked/answered.

As for those walls of text that constantly get quoted in replies I know I am NOT a big fan of those. I can understand the reason for them but they are extremely distracting and I for one hate seeing 20 lines repeated just for a one line comment. Part of the reason for the excessive quotes is probably laziness combined with difficulties editing content when posting from mobile devices; it's not too hard to hit "quote" and then type up your one line reply but it is far harder provide a simple identifier to what you are quoting. Nested quotes are some of the worst when all you really want to reply to is simply the outermost post.