Mainly if they have a lot to say in a single post and, out of courtesy, put them into hiders so that other players won't have to scroll through a veritable wall of text for something that doesn't really pertain to them. A second reason also has to do with that last bit; some post just doesn't really have anything to do with others, so they're put into hiders so that you can chose to ignore them, though I'd still suggest you read through them anyways since you never know what might be important.