It's not unnecessary if ES is just one of the endpoints. Kafka shines because it can accommodate a ton of consumers - so you can write once to kafka and then use it populate ES, databases, whatever. Furthermore, what happens if you need to re-index (say, if you update a mapping to an existing object)? It becomes trivial to reindex by replaying all the data from Kafka into ES thus saving you a lot of time.

If you are just dumping into ES, then yes, probably not the best tool (though it wouldn't necessarily hurt) - just use the HTTP API for that. However if you want to build a robust pipeline for multiple services or think you'll be needing to scale the feed into ES, Kafka is useful.