Even though school clubs in Japan are not mandatory, there is some pressure from the social grid (e.g. fellow students, teachers) to participate in some kind of after-school activity, be it in sports or arts. Colleges also have clubs. As a comedy element in anime, some students have joined too many clubs, and are thus constantly stressed out. There is also the element of social commentary, making it clear that you have to join a club, if you want to be part of the class. Since self-founded clubs need at least 3 members, the desperate search and advertising for new members can be cause for hilarity in anime.