Non-relational databases match one of the largest use cases that exist; large volume OLTP workloads. Non-relational databases typically can handle far more load on reads and writes than relational databases due to their intrinsic distributed structure, meaning that if you have any kind of transactional workload, for example, a frontend to a busy store (such as Amazon.com), non-relational databases are incredible. You can also back these frontend non-relational stores with relational ones but at much smaller scale in order to handle the OLAP requirement where relational databases shine, allowing for massively flexible queries when needing to pull data for reporting etc.