I also use SponsorBlock for YouTube, which skips sponsor segments in YouTube videos (and optionally other kinds of segments like intros, self promos, etc.) it’s crowd-sourced for identifying the segments but for almost all the videos I watch someone has already marked at least the sponsor segments
For me (I use Kavita) it’s because I want to be able to just pick up whatever device is in front of me at the moment and pick up the book where I last left off even if it was on another device