Terragrunt shines in cases where you have independent sets of Terraform state, especially if they are dependencies/dependents of one another.

For example, say you're using Terraform to manage AWS resources, and you've provisioned an Active Directory forest that you in turn want to manage with Terraform via the AD provider. Terraform providers can't dynamically pull things like needed credentials from existing state, so you end up needing two separate Terraform states: one for AWS (which outputs the management credentials for the AD servers you've provisioned) and one for AD (which accepts those credentials as config during 'terraform init').

Terragrunt can do this in an automated way within a single codebase, redefining providers and handling dependency/dependent relationships. I don't know of a way to do it in pure Terraform that doesn't entail manual intervention.