This exposes the TransformStreamDefaultController class on the global scope. This class already exists and can be accessed using code such as let TransformStreamDefaultController; new TransformStream({ start(c) { TransformStreamDefaultController = c.constructor; } }); This change makes such code unnecessary as now TransformStreamDefaultController just exists on the global scope.
Possible uses for the exposed class include monkeypatching properties onto TransformStreamDefaultController.prototype, or feature-testing existing properties of it more easily. (Note that the class is not constructible, i.e. new TransformStreamDefaultController() always throws, so that is not a potential use.) However, these uses are fairly esoteric. In practice this is mostly a spec-conformance fix motivated by consistency.