What is accomplished by producing data to a topic with a message key?

When a message key is specified in Kafka, the producer uses a partitioner (typically the default hash-based partitioner) to deterministically map all records with the same key to the same partition. Kafka guarantees order within a partition, so this enables per-key ordering.

From Kafka Producer Concepts:

“If a key is present, the producer will always route records with the same key to the same partition. Kafka preserves the order of records within a partition.”

This is essential for ordered processing and join semantics.