Besides side-stepping the burden of creating complex testing environments, progressive delivery provides a number of benefits over testing in isolated environments:
We reduce the difference between testing and production to near-zero, which gives us higher quality results. Monitoring new versions as they're rolled out allows us to automate promotion and rollback of new versions based on testing and metrics. Our tests automatically reflect real users, which fills in gaps we might have in our test cases. Gaining confidence in the resilience of production allows us to iterate faster instead.