Forum Runner is an app that makes browsing forums MUCH faster on mobile devices. It's not that the web browser on mobiles can't render this forum. It's that the forum isn't designed/optimized for mobile access. Forum Runner provides that optimization. A plugin gets installed on the server, and allows the app to retrieve forum information in a much more compact data format, saving mobile data usage and making transfers faster as you don't get all the extra HTML baggage. And then it also presents the forum posts in a manner that makes better use of the smaller screens that mobile devices have, so you aren't doing so much scrolling around.
Another benefit is that it supports push notifications, so if you've subscribed to threads/subforums , you can be notified right away anytime someone posts to them via the Forum Runner app, rather than have your E-mail get spammed with 'new post' messages.