RocksDB is used for several (internal) reasons (as you mentioned already for example its performance). Conceptually, Kafka Streams does not need RocksDB — it is used as internal key-value cache and any other store offering similar functionality would work, too.
Comment from @miguno below (rephrased):
One important advantage of RocksDB in contrast to pure in-memory key-value stores is its ability to write to disc. Thus, a state larger than available main memory can be supported by Kafka Streams.
Comment from @miguno above:
FYI:
"RocksDB is not written in JVM compatible language, so it needs careful handling of the deployment, as it needs extra shared library (OS dependent)."As a user of Kafka Streams you don’t need to install anything.
Using Kafka Streams DSL, as of 0.10.2 release (KAFKA-3825) it’s possible to plug in custom state stores and to use a different key-value store.
Using Kafka Streams Processor API, you can implement your own store via StateStore interface and connect it to a processor node in your topology.