there are many reasons to need something like pgbouncer.
1) not all workloads lend themselves to backend pools
2) not all backends can afford pooling
3) you don't always control your backend
4) you might have a lot of different backends connecting to the database. it's much simpler to just tell them to connect to pgbouncer.
5) you don't want to trust backend team to not bring postgresql down with many simultaneolus connections.
6) backend pools often misbehave
people don't wake up one day and just add another cog to their infrastructure randomly.