On the small scale, no, it's not pay-for-what-you-use. Imagine though you're an enterprise, running a large K8S cluster with lots of workloads inside. By using Knative and scale-to-zero, now you can pack more lightweight workloads into the same cluster resources, because the pods scale down when they're not actively being used. It gives you (as the cluster operator in your company) the ability to run your cluster the same way serverless works in the cloud.