Added motivation / why section to the IaC docs

doc/user/infrastructure/index.md

@@ -6,6 +6,13 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 # Infrastructure as code with Terraform and GitLab
 
+## Motivation
+
+The Terraform integration features within GitLab enable your GitOps / Infrastructure-as-Code (IaC)
+workflows to tie into GitLab's authentication and authorization. These features focus on
+lowering the barrier to entry for teams to adopt Terraform, collaborate effectively within
+GitLab, and support Terraform best practices.
+
 ## GitLab managed Terraform State
 
 > [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2673) in GitLab 13.0.