when you hit scaling issues with standard postgres. cdb has the same wire format so no code change would be needed. for mysql alternative, there is tidb but that one is not a single binary so i never used it.
A primary advantage is horizontal scalability. If you need more capacity, just add more nodes.
Additionally it offers better fault tolerance and zero-downtime online upgrades. For more advanced use cases it can be deployed in a geo-replicated topology and configured with partitioning for low latency transactions when confined to a single region while proving strong consistency and a single logical database globally.