Not OP; just a guess: old.reddit.com is a way to view and interact with Reddit using the original theme (or closely based on original look). A ton of tools were built for the site to look for data in a specific format. Many users likely utilize this “old” Reddit due to using legacy tools. It’s a way to get people onto the newer product so you can retire old infrastructure.

Basically, an enshittification decision by Reddit because they need to make more money.