For caching the query results you get from your database. Also it's easier to spin up Redis and replicate it closer to your user than doing that with your main database. From my experience anyway.
I think the idea is that if your db can hold the working set in RAM and you're using a good db + prepared queries, you can just let it absorb the full workload because the act of fetching the data from the db is nearly as cheap as fetching it from redis.
Of course? I'm not really sure what the original question actually is if you know that users benefit from caching the results of computationally intensive queries.