Add a "Why?" section to the readme. Fixes #22.

README.md

@@ -64,6 +64,36 @@ The last successful commit was a96d433b7e953bd8199eaf261b1dbd618ac05240
 
 If no successful workflow runs are found, the action will fail.
 
+## Why?
+
+As part of the CI pipeline for one of my projects, I wanted to generate a list
+of commit messages since the last successful workflow run. In order to do this,
+I needed a way to determine the last successful run of a workflow, which is only
+possible by using the GitHub API.
+
+Since I typically like to minimize my reliance on external dependencies, I
+initially created this action as a local action in the project repository that
+needed it. When it came time to implement a similar workflow in another project,
+I decided to move this action to its own public repo for reusability.
+
+Creating this action also gave me a chance to practice implementing, testing,
+and publishing a JavaScript-based GitHub Action. It was definitely a bit more
+work than I had initially anticipated, but I had fun learning the process and am
+happy with the result.
+
+When I initially created the local version of this action, I don't recall there
+being many other actions that provided this functionality. Since then, it does
+seem like there are some alternatives that are worth considering, such as:
+
+- [nrwl/last-successful-commit-action](https://github.com/nrwl/last-successful-commit-action)
+  (now deprecated in favor of
+  [nx-set-shas](https://github.com/nrwl/nx-set-shas))
+- [PatrikValkovic/last-successful-commit-action](https://github.com/PatrikValkovic/last-successful-commit-action)
+  (fork of the original `nrwl/last-successful-commit-action`)
+- [jmoll-hn/last-green-commit-action](https://github.com/jmoll-hn/last-green-commit-action)
+- [dharun-sahaj/last-successful-commit](https://github.com/dharun-sahaj/last-successful-commit)
+- [nickderobertis/last-successful-commit-action](https://github.com/nickderobertis/last-successful-commit-action)
+
 ## Contributing
 
 See [Contributing](CONTRIBUTING.md) for more information about how this action