Nothing. We use Terraform to provision a simple auto-scaling cluster with loadbalancers and certs, does exactly the same thing but there is no Docker and k8s. Few million lines less Go code turning yaml filed into seggfaults.

Consistency and standardized interfaces for AppOps regardless of the hyper-cloud I use. Kubernetes basically has an equivalent learning curve, but you only have to do it once

They operate at different layers. K8s sits on top of the infrastructure which terraform provisions. It's far more dynamic and operates at runtime, compared to terraform which you execute ad-hoc from an imperative tool (and so only makes sense for the low level things that don't change often).