As a creator (series, cuz no shame =D), a lot of people I subscribe to are for the social aspect of it. There's a lot of artists that have series running BUT also showcase a lot of their art.
Also, I might be biased towards people with lower sub counts (you know who you are). They tend to be more active, responsive, and generally fun people who engage in my work as well.
There's a ton of series here worth subscribing to that have thousands of subscribers (some that I have subbed to as well), but I will admit that there's some popular series I JUST don't bother subbing, mostly because they're easily accessible on the front page and featured list anyway. I'll even less likely comment on their episodes because, let's face it, we know when it's not gonna be read.
Yes, I tend to put a lot of popular series I subbed to on low priority and will someday binge all the latest updates, but for the active users still in the works of building a following that I enjoy, I'll put forth the effort to like and comment every new update, especially if it means pushing them up the "trending/popular" list.