Presumably the web tier does slightly more than just reverse proxying. For example, it could build (render) pages based on an internal RedditAPI it queries. This RedditAPI (application layer) would then basically be a distributed database front-end with some state, like user sessions.

Seperating it at that point allows the web tier to offload much of the work (mostly rendering), while keeping it stateless, thus allowing effortless scaling of that tier.