Before they started nerfing it you could set it up to show you 100 posts at a time, and reading the comments in a post would load them all at once so you could click down a list of posts across multiple subreddits, open the ones you thought might be interesting in new tabs, then read at your leisure. You filtered by just scrolling more, and could do this without creating an account.

Now they do all this "for safety" stuff, where you have to use Baby's First Web Interface to look at a few comments that are so milquetoast that it'd be hard to call it a discussion at all. I'm sure it's just coincidence that the changes also force click after click after click to see more posts which can be used as proof that it's a user engaging (I guess?) and not a robot.

It used to be seriously nice to read through comments, come across a word or concept that you didn't really understand at the time, then go back and search the full text of all the comments for that word again. Folding comments killed that.

For some, especially veteran users, the UI is more familiar. It's also more predictable and less prone to bugs than the new UI, and especially the glitch-laden official app.

Also, there's less ads. I don't see any ads on old.reddit but that might be uBlock Origin at work.