This is how I feel, and why I think the sub for sub threads have been a net positive for me. It helps me connect with other writers and artists. It introduces me to some cool stuff I might not have found or checked out otherwise. And it has gotten me some active readers/likers/commenters who seem to genuinely enjoy my novel.
As for the marketing aspect, I think others have raised some good points, but I also think sub for sub can be a useful part of a more comprehensive marketing strategy to get readers and the important likes and comments for the visibility algorithm, which in turn helps you reach more people, particularly for new works by not already established writers. But while I've been here awhile as a reader, I am new here as a writer and I don't have all the answers or know how everything works.
I think people who haven't used it haven't heard enough from people who do what positives there are, and seem to assume it's a marketing/growth tool only, and that it's the only marketing/growth tool people who do it are using.
I'm not here to become famous or rich, and I know that my novel is pretty niche (novel on a comic site/popular novel category but not falling into the most popular subgenres and styles/dark, urban and gritty) but I have had a blast writing it and am thrilled to share it with some readers who have come to like it. Sub for sub has helped contribute to that.