In addition to what others are saying (you can be a writer and a reader and whatnot), I think where the forums really help out is that they when you get engagement from other writers it gives you the boost to be noticed by non-writing readers. I find this works on several writing sites I've been on--most of which I've left for other reasons. Very anecdotal, so take it with a grain of salt.
For example, as a reader I might see some stories have 0 to 20 views, and maybe 1 comment. If that's the average for most of the stories, then anything with, say, 100 views and 10 comments is likely pretty good. There's something about it that is compelling people to engage.
Most of these views and comments might be from fellow writers who are paying it forward, or fulfilling their part of some reading swap, but I'm not necessarily thinking about that as a reader. I just see engagement, and a higher view count than normal, so I start reading it, too.
Now that the view count and engagement is higher, the algorithm sees that this story is doing well, and brings it into more notable categories like What's Hot or Trending, or whatever [insert any site] calls it. This is where non-writing readers end up finding it because, hey, it must be good to make it there.
That's not to say that's exactly how the algorithm works. Writing sites generally don't release how their algorithm works, for fear of others playing the system, but that's often how they seem to work (even non-writing sites), and fellow writers helping each other get out of the beginning pit of "the algorithm doesn't even know I exist" can give that boost needed to be recognized by other readers.
Readers will also lurk forums and other writing chat areas, the same way readers will lurk stories but never comment even if they are actually reading it thoroughly as any other commenter or reviewer. 
I also think the audience for Tapas novels is, in general, mostly writers. Tapas novels are still relatively new, and a lot of readers flock to more well-known reading sites, like Wattpad (even then, it can be just as hard to get anywhere on Wattpad, and those that have a view count in the thousands have often been on there a very long time. Wattpad has carefully designed their site to make it look like each story gets way more reads than they actually do). Maybe one day that will change, who knows. I like the Tapas community enough I want to stick around and watch it grow. ^^
