In regards to the "purpose" of having more subscribers, there are algorithms to track how many people like and comment on your pages that can move your comic up to some of the more visible sections on the front page like "new and noteworthy" or "trending" so you get more attention. From a purely numbers perspective, the only other milestone is having to have 2,000 subscribers to apply to be a premium comic.
Readers do not get notifications for a comic's new updates unless they are subscribed to it.
If a reader adds your comic to their subscription list, you get a notification about it. There is no "private list" where they can subscribe without the creator knowing about it or it not counting toward the subscriber numbers.
The subscriber/reader/liker/commenter ratio is always going to be like that. People will always subscribe to more than they read, read more than they like, like more than they comment, and comments are infrequent. A lot of people will subscribe to something with the intention of reading it and never get around to it, or wait until it has more pages up so they can read a lot at once, or subscribe just so they can share it with a friend. Lots of reasons.
Personally, I have about 1,000 subscribers right now. Of that, about 700 actually read each new page, 70 people like each new page, and about 7 people leave comments. That's just the nature of Tapas.