Most Terraform users think modules exist to reduce duplication.
That is not the real problem.
The real problem is dependency control.
When your root module directly depends on multiple internal components, every change becomes dangerous. A small refactor inside networking or security can break unrelated parts of your system.
Module composition solves this by assembling child modules behind a controlled interface.
Instead of exposing everything, you expose only what the system needs.
That one decision changes everything.
Now your dependency graph is driven by data flow, not manual ordering.
Now your refactors stay inside boundaries.
Now your blast radius is contained.
This lesson shows how module composition actually works under the hood and why most implementations fail silently by leaking outputs.
If you are building Terraform beyond simple labs, this is where your design either stabilizes or collapses.