What is the purpose of a key in Kafka?

In Apache Kafka, the key is used to determine the partition to which a message will be assigned. Messages with the same key are guaranteed to be written to the same partition, ensuring the order of processing these messages. This is important, for example, when it is necessary to maintain the order of events for a specific user or object.

Example: if the key is the user ID, all events of this user will go to one partition and be processed sequentially.