(1) minikube, which I also maintained at Google. I saw how difficult getting started with Kubernetes could be even when you were able to create a local cluster. I also knew all the shortcuts that you could take to streamline local Kubernetes workflows (e.g., loading images directly into the minikube docker daemon).
(2) docker compose, which remains a great tool but had no analogue for Kubernetes. It also didn't have a concept of syncing for hot-reloadable projects or built-in file watcher.
(1), Draft by the Deis folks at Microsoft, which was a great tool but suffered from having a component that you needed to install on the cluster to work, where the skaffold architecture was purely client side (making it a lot easier to get started, and faster). I also wanted to make skaffold pluggable, so it supported helm, kubectl, docker, bazel, and other tools right from the start. It also had clear boundaries between build and deploy and ideas for CI and gitops workflows built in.
You can tell it to just sync certain changes (e.g. syncing a javascript file to a container running webpack).