If you handle database requests naively, every request to the database may open its own connection. This is a super simple approach but will exceed the amount of connections the database can or wants to handle concurrently.

One solution is to increase your app complexity and introduce a layer that manages connection pooling or queuing.

Or you can just keep your app naive and simple and put pgbouncer transparently in front of your DB. Even for multiple apps, so instead of every app increasing in complexity, reimplementing connection handling, you just have pgbouncer.