I do this for multiple reasons, one is that I find it easier to use Kubernetes as the backend for Patroni, rather than running/securing/maintaining just another etcd cluster. But I also do it for observability, it's much nicer to be able to pull all the metrics and logs from all the components. Sure, it's possible to set that up without Kubernetes, but why if I can have the logs delivered just one way. Plus, I prefer how self-documenting the whole thing is. No one likes YAML manifests, but they are essentially running documentation that can't get out of sync.
I will say that even though the StatefulSet pins the pod to a node, it still has advantages. The StatefulSet can be scaled to N nodes, and if one goes down, failover is automatic. Then you have a choice as an admin to either recover the node, or just delete the pod and let the operator recreate it on some other node. When it gets recreated, it resyncs from the new primary and becomes a replica and you’re back to full health, it’s all pretty easy IMO.