Storing the data directly inside the application still means you need to store it somewhere, likely a SQL database (such as PostgreSQL). These databases are insanely well engineered and very very fast, but compared to a key value store such as Redis and Memchached they are comparatively slow and resource hungry (because they are optimized for different things).

So if you can fetch some cached data from a Redis key, even if on the same machine, it will cost you significantly less than querying a relational database.