Kubernetes orchestrates your container workloads over a cluster, which consists of virtual/bare metal machines(nodes). This means you can tell the kubernetes API "I want to run a container workload" and it will be started on one of the nodes that form the cluster, unlike e.g. Docker, where a docker daemon belongs to a specific node. If you remove the node your workload is running on from the cluster the workload will be rescheduled on a different one, or if you have a new image version it will start the new container, wait for it to become healthy and ready, and then route requests to it. And if you want to send requests to your workload kubernetes allows you to define standardized abstractions to easily route them to your workload, irrespective of the node it is running on.

It allows you to stop caring about the individual machines, and just treat them as combined compute, which starts mattering if you leave a single machine setup and need to start thinking about scaling in and out and gluing the individual parts together. Then you have known abstractions to do it.

Of course you can do everything kubernetes does using a bespoke solution, and the concepts aren't new, but having a widely supported technology has a lot of advantages and creating something with even half the feature has a high chance of just being worse.