My guess is that they want to get more subscribers. I mean what other reason is there?? If its to stop plagiarism then wouldn't Friends Only be better?

Well there's now a thing where you can view the story even though it's set on subscribers only but anywho: Some authors have said they've been plagiarized numerous times and only have a handful of readers they can trust: That's perfectly logical if you ask me. Some use it, so only their subscribers can view it and that's okay to ^^ So far those are the only reasons I've heard though.

I guess, authors will try to make their description/foreword really interesting to read, so the reader will click further more into the story and they will subscribe to read more, so they could get more subscribers. And if someone's advertising, of course they would want to put it on "Subscribers Only" because there are so many people on AFF that will click on that advertised page. Hope that helped. ^^

I believe it's a give or take situation. If you want subscribers... lol, I only put things on subs only if I think people aren't necessarily going to read it...

hmm I have not tried it, but I often see others do it a lot

I've only done that for one story so far and the story is kind of private in a way so I wanted some way of controlling who viewed it... That's my reasoning.