- Makes it possible to go from zero to fully running k8s integrated components in 5 seconds by just running 'helm install --repo https://example.com/charts/ mynginx nginx' (very useful: https://artifacthub.io/)
- Gives the ability to transactionally apply k8s configs, and un-apply them if there is a failure along the way (atomic rollbacks)
- Stores copies/versions/etc of each installation in the server so you have metadata for troubleshooting/operations/etc without having to keep it in some external system in a custom way.
- Allows a user who doesn't know anything about K8s to provide some simple variables to customize the installation of a bunch of K8s resources.
- Is composeable, has templates, etc.
So basically Helm has a lot of features, while Kustomize has... one. Very different purposes I think. You can also use both at the same time.
Personally I think Helm's atomic deployment feature is well worth it. I also love how easy it is to install charts. It feels a bit like magic.