After seeing two Facebook posts of quite different length each having a "more" link that revealed only a few more words, I came to the conclusion that it has nothing to do with a clean layout and everything to do with the algorithm getting a stronger signal from the user that they're actually reading a given post, not just stalling their doomscroll for unrelated reasons such as an IRL distraction.

If I want to see a little less of a certain type of post, but not to the extreme of unfollowing a given poster, I'll try to avoid clicking "more" in hopes of training the system.

I have worked on sites that do this (sorry)

Filtering spam from engaged traffic is a big one. News sites tend to do it in the vein of TikTok: users can quickly scroll to the next post if bored, instead of leaving. Some e-commerce sites do it to product descriptions and their ilk for the modern equivalent of keyword stuffing.

It may well be in some part because of what you say. But I _think_ one of the points is to allow playing video with sound. Current popular browsers only allow websites to do this after a user-initiated action on the site, such as a click. I've seen cookie bars misused to trigger autoplaying videos in the past in this way.

See, e.g., https://developer.chrome.com/blog/autoplay#new_behaviors

For some sites, it may be a performance optimization, to not load content users aren't going to see.

It depends. Check for event listeners on the button if you want to know the answer for a specific site.

I think it's often used in metrics to count as a reading.