Maybe because it can then also cater to audiences who do not generally visit such sites or reading long-er articles.
or maybe it recapture reader's attention, there recently (or always have been) this idea that if you don't like what you are reading you can move on and not waste your time especially with books. But most people generally want to know how that thing ends. So they make a quick decision how little time they have to spend based on the percentage of the reading material remaining.
Either way a reading progress bar is a UX improvement and has nothing to do with the intellect of the reader.
or maybe it recapture reader's attention, there recently (or always have been) this idea that if you don't like what you are reading you can move on and not waste your time especially with books. But most people generally want to know how that thing ends. So they make a quick decision how little time they have to spend based on the percentage of the reading material remaining.
Either way a reading progress bar is a UX improvement and has nothing to do with the intellect of the reader.