SubUnit's http://www.robertcollins.net/unittest/subunit/ purpose appears to be to manage testing via subprocesses.
Consequently, it can run external tests not in Python, it can fork a subprocess to manage the Fixture in an isolated process.
Adding this subprocess execution context extends the Test Runner implementation of the built-in unittest module. This can be useful in contexts where the Fixture includes Singletons or connection pools or other per-process design features.