add clarity on the why.

Author: wparad

README.md

@@ -3,9 +3,12 @@ This is a GitHub Action to automatically rebuild pull requests when the pull req
 
 Review the GitHub action in the marketplace: [GitHub Rebuild PullRequests](https://github.com/marketplace/actions/github-rebuild-pull-requests)
 
-### How does this GitHub action work?
+## How does this GitHub action work?
 When a branch in your GitHub repository is updated, this GitHub action automatically finds open pull requests that target that branch and forces a rebuild for them.
 
+## Why?
+This GitHub action exists because GitHub will not re-trigger builds. That means when a PR targets a branch and that branch changes, that PR is now out of date. That meas tests are not rerun, validations are not rechecked. Breaking changes will be present and not validated. By adding in this action, those PR will be automatically rerun.
+
 ## Usage
 In your GitHub action workflow add the follow step.