When new features come available in PHPUnit you will not always be able to use them directly due to several possible reasons.
When you do want to use these new features, but your tests need to be able to run on older versions of PHPUnit (e.g. because of older supported PHP versions) this compatibility package might come in handy. It aims to implement new features of PHPUnit supporting older versions.
Instead of extending the PHPUnit\Framework\TestCase class, you'd simply extend the Procurios\TDD\PhpunitAdapter\PhpunitAdapterTestCase class and you're done!
This package only extends the PHPUnit\Framework\TestCase class. You will be able to use any method available in phpunit 5.5.0.