Sidecar container pattern allows to modify application functionality  and capabilities without changing the main container. It is done by deploying additional containers in the application pod.

Adding this capability is requested by our internal and external users:

1. Support - to run debugging tools

2. Engineering - get the logs in one place

3. Customers - run monitoring sidecar containers of their choice

Example of the implementation:

https://github.com/GoogleCloudPlatform/spark-on-k8s-operator/pull/478