Managing one app on one machine (physical or virtual) is a significantly different problem from needed to manage dozens or hundreds of applications/servers.

When you need to automate the entire deploy process, possibly scale up or down the number of servers/services running based on demand, or to run the same operation across hundreds of servers: that is when you need Kubernetes. Trying to do any of these tasks manually when you're dealing with a large number of machines/services/servers becomes unmanageable very fast.