Topic encryption, policy-enforcement, multi-tenancy, audit and much more.
Proxies are a powerful architectural pattern which are widely used for other application-layer protocols, such as HTTP.
They could solve a number of problems organizations have using Kafka. While some organizations running Apache Kafka have written Kafka-aware proxies, they have tended to remain closed-source and do very specific things.
Kroxylicious is an early-stage project which seeks to lower the cost of developing Kafka proxies by providing a lot of the common requirements out-of-the-box.
This lets developers focus on the logic needed to get their proxy to do what they need.