IaC is a silly term. Infra is mostly hardware, which by definition is something else than software.

But hardware needs to be configured. And load balancers, firewalls, clusters etc. are perfectly suited for the declarative style of the Ansible/Puppet/Chef type of tools. That is what people usually mean by "IaC", as silly as it may be.

Those tools really shine when used end-to-end. The defintion of an application can contain which ports need to be open towards backends, database users, and health check parameters for the load balancer. The system can then declaratively sort out the respective templates, and configuration really lives in one place. Shared secrets are defined exactly once, and rotation is deterministic across the entire environment.