A few reasons.
I like finding the actively updating novels that I can follow in the same genre as mine. Their readers see my comments. My readers see theirs.
So, I do sub for sub with those writers. A lot of those works I continue to follow. From similar works, I would rather pick those by the writers who could become interested in mine. Some, I just want to support because they are good stories, not getting enough attention.
You need a min sub count to start showing up on the app section in Fresh. So, it is beneficial to get a few first subs right away, to have more eyes on the story from the app ASAP
In certain genres, sub for sub will help you get into popular section that increases the chances to be noticed by readers.