It's pretty useful when you have some distribution layer (i.e. some pubsub system)
Consider 10-15 applications running on a host, and all of them are listening to data being distributed by another service. Instead of all of them opening a connection to that service, instead they would all be connected to this sidecar, and the sidecar would merge the distribution of data (and subscriptions) to the pubsub system
Consider 10-15 applications running on a host, and all of them are listening to data being distributed by another service. Instead of all of them opening a connection to that service, instead they would all be connected to this sidecar, and the sidecar would merge the distribution of data (and subscriptions) to the pubsub system