Why use this over kubernetes, if it's the same but less memory why not only use k3s ans drop k8s
You have to think of k8s as more of a specification of what can be done with multiple implementations. k3s, kind, microk8s, minikube are all implementations of kubernetes that you might want to try out.
k8s is much like the original IBM pc or a linux distribution. There are a bunch of interchangeable parts that you may land on depending on your needs.
Why use this over something else? I've found k3s to be really simple to setup and upgrade, which is why I use it for my home lab.