I'm on old.reddit.com too and I use the mobile app (including the 3rd party ones back when they existed) for one primary reason: Two windows I can quickly switch back and forth on. On my phone I use Reddit to look up things. I can have a Reddit thread on one window and a Google search on the other and go back and forth. In a browser switching tabs back and forth is painful, often reloading pages, losing the spot in the browser, having this url bar and top bar taking up tons of screen space.