I should clarify, plProxy is responsible for the transparent horizontal scaling. pgBouncer is a proxy that allows you to run all of this using a minimal number

I should clarify, plProxy is responsible for the transparent horizontal scaling. pgBouncer is a proxy that allows you to run all of this using a minimal number of PostgreSQL backends, which is essential because of the amount of inter-process communication locking and information sharing in PostgreSQL. pgBouncer basically manages a smaller pool of active backends reducing the overhead in running high transaction velocity servers.