Because I've been plagiarized before. So I attempt to lock down my popular stories or the ones who are most at risk of being plagiarized, even if it's just a little. It brings me some safety, because I'll know where to start searching from if something mine ever gets out again. I know there's a website in which you just put the link of the story and then you download it even if it's locked but, I don't know. It just makes me feel a little bit safer.
Overall, I think it's stupid to ignore a story just because it's set to subscribers-only, because most people assume the author just does it for popularity. Who cares ? As long as the story is good for you, why would it bother you to press the subscribe button to be able to read it, you know... The story is the story, the author is the author.

I have the same reasoning as Wendy Wu. Not really towards for more subbies but towards more of if you like what you read then subscribe for more, kind of thing

Even I don't know why I do it. Hmm... maybe I do it because I feel insecure..? If they subscribe then unsubscibe. I would know that they dislike(?) That kind of genre. I don't like to force people or anything but it's really complicated to explain. Lol. Some authors maybe 'force' the readers. But for me, well, they could criticise me but if they wanna bash, well, why click the subscribe button, bash then unsubscibe(?) I don't think you'll understand. Btw, kamu dari M'sia?

Because there was the button that allows it.

Now with the preview for these things, subs will mean that those readers are committed the story. It gives an idea to the author how popular the story is, because view counts do not equal an actual reading of that story and/or chapter. As an author, it is encouragement (since most readers are silent and do not leave comments)

for some people, it's probably because they're greedy for subscribers so they're trying to "force" people to subscribe. for others, primarily popular writers (I use that term loosely because no writer on AFF is really THAT great), I believe it's yet another a way to limit plagiarism, and if they DO get plagiarized, there's a limited amount of people who could've done it.

Because they want more subscribers. So they think doing that will get people to actually stay and read their story. It kind of works because even if I did not like the story I do feel kind of bad unsubscribing