Relational databases are optimized for typical operations over data structured in tables. So, joins and records. However sometime you want something simpler - like LIFO queue - and wouldn't mind to have is faster. Redis allows to have this - the variety of data structures it has is much bigger than with relational databases. They (Redis and RDBs) both have their uses, of course. Ideally you would structure your system to use one of them where appropriate according to data requests.