There are various use cases of replication slots. The most simplistic use case is the one shown here. Data can be fetched from the server in the desired format and used to audit, debug, or simply monitor a database instance.

The next logical step is to take this stream of changes and use it for replication. Solutions such as bi-directional replication (BDR) are totally based on logical decoding because changes at the binary level would not work with multi-master replication.

Finally, there is the need to upgrade without downtime. Remember, the binary transaction log stream cannot be used to replicate between different versions of PostgreSQL. Therefore, future versions of PostgreSQL will support a tool called pglogical, which helps to upgrade without downtime.