PHAR-files are a great way to bundle code that can then be used like a binary. Therefore PHAR-files are often used for tools that can be included in a CI/CD setup. As they are self-contained archives they use their own autoloading mechanism and therefore don’t depend on your code. Which is great if you want to use them as build-tools because the tools dependencies don’t interfere with your project dependencies.
Imagine you’d want to use a build tool that requires PHP 7.2 with your legacy code that still needs to run with PHP 5.6… Most probably your composer require --dev awesome/build-tool would not work because of a dependency mismatch. Even though you might be on PHP 7.2 at the moment. Using the PHAR-file removes that dependency as all the required files are contained within the archive. And the PHARs autoloader takes care of getting the right files.