How do you test that a feature in your application actually works? Or that it is actually being used?
Usually, you’d perform lots of tests in separate environments, like a QA or Acceptance environment. Or you’d route a certain percentage of your production traffic to the new version of your site to test if and how usage of your new features works. This latter approach used to be very difficult. You had to implement and configure a load balancer to route some of the traffic to the new version of your application.