The clients actually acts as a cluster node and therefore has knowledge about the cluster state, its indexes and shards, because it receives notifications from it, once it joins.

This allows to execute operations on a specific shard of a specific index on a specific node of the cluster resulting in better performance than going through the HTTP interface.

It can be used to efficiently store big quantities of data, for instance logs, which then can be visualized with Kibana.

It's just unfortunate that Elasticsearch presents the problems mentioned in the article and which I also experience in production, because it has a series of plugins which makes it a good solution for specific use cases.