I suppose none of these clubs are really necessary per definition, but they make the process much more enjoyable. 
While in a book club you often end up having to make concessions in regards to what’s being read, I think there’s a couple advantages over ‘lone’ thread.
Since everyone contributes with ideas, you get exposed to a large variety of genres and styles.
Since there’s a majority poll, there’ll be a maximum of interested readers.
Since there’s a majority poll/it’s a general club, even people who’re not particularly interested in a given poll may end up reading along and participating by default. (I think especially at first when you haven’t read enough to really know what kind of writing style you like in Japanese it’s a really good thing to be exposed to a good variety)
Also I think it’d be good to have a place for nominations that get excluded simply for being to long.
Also, as @Azmo said, as a place for gathering/idea collection etc so you don’t have to search for your target audience from zero every time.