📝Key-value stores can be used as database storage engines
Key-value stores provide primitive API but are very fast and solve persistence. They are good ground for implementing higher-level databases (document, graph, etc.) and some databases are implemented in this way.
Examples
CoackroachDB uses RocksDB as its storage engine
ArangoDB uses RocksDB as a default backend starting from version 3.4.
more examples at RocksDB > Integration - Wikipedia