Yes the big bag of data which is generally the value in the kv store is a json (either as a serialized string or a map if it is natively supported by redis).
Yes, the usefulness of SQL always is the join or group but with something like session, the idea is to just use it to dump values into the store which you don't want to reach into the database every time. So different teams will want to put in their own keys and data into the session object which means your DB session store becomes extremely difficult to maintain over time.
On the other hand, the joins and groups based on this can be handled later in time than in the req-response cycle itself.
Yes, the usefulness of SQL always is the join or group but with something like session, the idea is to just use it to dump values into the store which you don't want to reach into the database every time. So different teams will want to put in their own keys and data into the session object which means your DB session store becomes extremely difficult to maintain over time.
On the other hand, the joins and groups based on this can be handled later in time than in the req-response cycle itself.