As an author!
I keep mine set to subs-only for two reasons.
1) I started off with it because I would see X amount of reads, but no comments, no subs, etc. I believe that it is unfair to the author and their hard work in providing content for fans to read to not subscribe - we're not paid by any means, but it is a lot of work to have a cohesive story! And yet only about 1 in 20 readers comments (using my stories to find averages). So in a way for me, the subs only is keeping a record of who has "checked out" my book from the library, or "bought" it from the store, and giving me back slight rewards for each person that has. The simple act of clicking "subscribe" is their "payment" in a way for my hard work.
2) I used to only have stories that were completed be subs only, but then I had the same story plagiarized and posted elsewhere 10 (TEN!) times and said eff it, I'm making everything subs only. I keep tabs on everyone who subs to my stuff and it helps me to have a sense of security for my stories.

As a reader!
I like to see when other authors have their stories as subs only - I will generally look at the first page (even if it is subs-only, you can see a preview without being able to read it all), and if like what I see, I will subscribe and stay subscribed. Even if I lose interest in the story years down the road, or if it is never completed, I stay subscribed just as a courtesy to the author. I make use of the labels that AFF provides and use those to sort my subscriptions, just as I sort things on my own massive bookshelves. I will NEVER read something without subscribing, no matter the length or quality - if I read it, I will subscribe.

To me, it all boils down to this and this alone - I would never go into a bookstore, sit down and read a chapter, and then come back each day to read another chapter until I am finished and yet never buy the book. It's stealing, it's rude, and it's wrong. And I feel like those who don't subscribe to what they read on AFF are doing just that, and so my setting is ensuring that they can't with mine.

i do it mainly for short stories and finished stories. it’s mostly to keep a following for the story. i know that’s pretty selfish of me but you gotta do what you gotta do. however, if it’s a long, regular story then i leave it open for all. :))

Like louieistrash, my stories are only subscribers-only after completion, simply because I don't want to bother people with updates. Especially if they're waiting for the full story to be posted before reading. Subscribers-only lets me monitor my readers, see who's still reading/following my work, and how many people are actually interested after looking at it while it's ongoing. In the past, I used the feature because a reader and friend recommended it to me. Plagiarizing stories on AFF wasn't unheard of back then. I also did it for the KPs and as a reminder, since some forget to subscribe so they end up asking around for the stories they didn't subscribe to xD And as a former reviewer with a whole review shop, subscribers-only protected requestors' stories, prevented spoilers to bypassers, and again, reminded people to subscribe so they can see more of the shop and won't fail to pick up their reviews.

As a reader, subscriber-only forces me to subscribe before I can snoop around to see if I'd want to keep reading or not. I normally don't subscribe in case I did not like the story and will feel guilty later for unsubscribing. So unless the description/summary is very intriguing, if it's an ongoing fic, I don't subscribe. From the readers' perspective, I can also see it scaring potential readers away. Not everyone uses an account, there are a handful of silent readers out there, and some can't be bothered to create an account just to read fanfiction. Subscribers-only stories can also come off as the author being needy and greedy for subscribers, readers, etc. And not every author interacts with their readers so that might add to readers believing authors write just for fame and so on.

I used to do it so my friends wouldn’t be able to read it completely. Now I leave it open for anyone to read.

I do it for the kp, not gonna lie xD it's also a good way of seeing how many ppl show interest in the fic. Though this can be a double edge sword. Sometimes no one subs xDDD my short fics are friends only. Wanna do something once for friends only.

As someone who leaves a story to subscribers-only, I feel the need to comment. Hahahaha.

I usually put my stories on "subscribers-only" once it is completed. It's for me to monitor who's reading the stories too - not really for the purpose of interaction but just to have some "sense" or "clue" if my stories still attract readers. Sometimes, when I get a subscribe from a story I wrote/completed years ago, it gives me some sort of awe that that the story still piques people's interest, and also when I get a subscribe from a story that I wrote which has a tragic ending, it makes me wish goodluck to the new reader. I'm also writing in another site and when I see a subscribe from the same username in that site, I go "oh she's also here." Hahahaha. I guess it's my personal measure of how many read/are still reading my stories because "subscribe" goes to the push notif immediately. I think also that it represents more accurate data/number of unique readers of my completed stories - not that I am completely moved by such numbers, but it's just nice to know which stories I wrote people read more. Haha. It's like an FYI for me now.

I also don't think that "subscribing" is such a burden to do to the readers? It just takes a click, they can click unsubscribe after if they want too. It doesn't even take any karma points. I don't think it carries such heavy meaning like upvoting too. If it's about issues on AFF showing which stories you are subscribed to, there's already a privacy setting for that, I believe.

In my case, if they subscribe, they wouldn't be bothered with any updates because it's only the completed ones that I turn into "subscribers only." Plus in my case as well, I think it also puts "readers after the story is completed" in equal footing with those "readers before the story is completed" because the former would also need to subscribe to find out what happened but minus the waiting time for updates askdhasjd.

When I was younger, I was kind of pressed on getting more upvotes and subscribers, but after years of being in AFF, I am also on my way of learning how to write for the sake of writing. But as a 'words of affirmation' person, I can't erase the fact that I still get motivated with comments/upvotes/subscribes. So I what I'm trying to say is numbers (subscribed/upvotes/comments) are not really a great factor if I begin/continue/discontinue a story, but I still appreciate any form of affirmation asjdahskdahsj.

I hope I make sense somehow. Haha. I am also interested to see others' thoughts on this!