Jussi is a reverse proxy that acts as a caching front end. It forwards the requests sent to it to the proper channel, such as a full RPC node or the SBDS client you mention. SBDS is the blockchain parsed into a local database for faster querying.

Having a caching layer in front of the DB means repetitive requests for the same information do not hit the DB multiple times. The first request returns a result and stores the request/result pair in memory (Redis). Any subsequent requests for the same information return faster because the data is prepped in memory and does not have to be searched thru the DB.

This means the DB can serve other requests faster and requires less resource demand.

Essentially, Jussi is just an NGINX reverse proxy that uses REDIS to cache requests in front of a DATABASE (SBDS / Steemd RPC)