Terragrunt is a lightweight WRAPPER that adds extra features for maintaining DRY configurations, dealing with many Terraform modules, and MANAGING remote state.

Following are the use CASES of Terragrunt:

• To Keep Our Background Configuration DRY (Don’t Repeat Yourself): By setting your backend configuration once in a root location and inheriting that information in all child modules, Terragrunt helps you to keep it DRY ("Don't Repeat Yourself").
• To Keep Our Provider Configuration DRY: It might be difficult to unify provider configurations across all of your modules, especially if you wish to alter authentication credentials. You may use Terragrunt to refactor common Terraform code and keep your Terraform modules DRY by using it. The provider configurations can be defined once at a root location, just like the backend configuration.
• To Keep Our Terraform Command Line Interface arguments DRY: In the Terraform universe, CLI flags are another typical source of copy/paste. It can be difficult and error-prone to have to remember these -var-file options every time. By declaring your CLI parameters as code in your terragrunt.hcl settings, Terragrunt helps you to keep your CLI arguments DRY.
• To Promote Terraform modules that are immutable and versioned across environments: Large modules should be considered hazardous, according to one of the most important lessons we've learnt from building hundreds of thousands of lines of infrastructure code. That is, defining all of your environments (dev, stage, prod, and so on) or even a huge amount of infrastructure (servers, databases, load balancers, DNS, and so on) in a single Terraform module is a Bad IDEA. Large modules are slow, insecure, difficult to update, code review, test, and are brittle. Terragrunt LETS you define your Terraform code once and then promote a versioned, immutable "artifact" of that code from one environment to the next.