There been a lot of CI/CD systems before ArgoCD (Jenkins, Teamcity, CircleCI, etc.) What added value ArgoCD brought?

Simply said, ArgoCD is CD, not CI. We still need CI systems. Secondly, ArgoCD is running on Kubernetes, it's part of its ecosystem, as opposed to some other CI/CD systems. Finally, ArgoCD was built specifically for Kubernetes, not other platforms and systems.

Easier to explain the need for ArgoCD by direct comparison to another system that can do CD. Let's use Jenkins for this.

With Jenkins, you need make sure to install k8s related tools and set access for commands like kubectl. With ArgoCD you simply need to install it in your namespace but no need to install additional tools as it's part of k8s.

With Jenkins, managing access is usually done per pipeline and even if set globally in Jenkins, you still need to configure each pipeline to use that access configuration. With ArgoCD access management to k8s and other resources is given as it runs already on the cluster, in one or multiple namespaces.

With Jenkins, tracking the status of what got deployed to k8s can be done only as an extra step, by running the pipeline. This is because Jenkins isn't part of the k8s cluster. With ArgoCD you get much better tracking and visibility of what gets deployed as it runs in the same cluster and the same namespace.

With ArgoCD it's really easy to roll back to a previous version because all the changes done, are done to git which is a versioned source control. So it's enough to get to a previous commit for ArgoCD to detect a change and sync to the cluster. Worth to mention, this point specifically is true for Jenkins as well :)