It's a mix of things for me.
Accountability is one. If I know there are people waiting for the next installment, I'm more likely to actually work on releasing it.
Ease is another. Posting on a site like Tapas takes slightly less effort than putting together an ebook with an isbn and a cover, and making a pdf and full cover for the print version...
Also, this format feels low risk to me. Not sure why. Maybe it's because I'm only posting a little at a time?
I haven't gotten much feedback here yet, but I just started posting this past Friday. lol Hearing what people think is good, but you have to be strong enough to keep true to your vision while also acknowledging your faults. Take everything with a grain of salt. 
Writing is art, and I believe that art is created to help people see the world differently. Sometimes, it helps just the creator, and sometimes that's enough, but if the creator shares their art, that art can then help others.
I say, if you have a story you want to put out in the world, do it!
It could be the story someone out there has been wanting and waiting for.