We have been underestimating the potential of unit tests because of their name.
In fact, unit tests are general-purpose helper scripts that can be run at any time by the UnitTest test runner.
The cruft of having to create subclasses of UnitTest.TestCase usually obscure this simple fact. Leo's @test nodes do away with the blah-blah-blah of unit testing, so this Aha is much easier to see and exploit in Leo.
That's all there is to it.