For mobile I think usually it's so it can handle error cases more gracefully. How people expect something should act when the network goes away and it's a web page is often different than when it's an application.

The Reddit thing is also mixed with the fact that Reddit's mobile web interface is complete shit. You can't even collapse threads (or it's so hidden as ot be useless), which is something the desktop web interface does. It might not be the best example of why mobile users want an app, as it's functionally worse using their mobile web version, probably on purpose to force you to their app so they can monetize you... and now we understand why they did what they did and why people didn't want to use their official app, as the way they monetize you is to inconvenience you with more ads.