IMO this is one of the greatest new features we've been spoilt with for a long time. It allows me, as an automotive renderer, to render the car with a HDRI and an asphalt ground (which I set to "invisible to camera") without actually seeing anything else than the car in the render, apart from in the reflections. And, best of all, it's done all at once - the object and its materials gets rendered at the same time as the alpha
This, together with the shadow catcher feature, allows for easy integration with a backplate.
handy alpha.jpg [ 780.77 KiB | Viewed 9408 times ]
For example, you could render an entire room with an ugly HDRI - with the alpha box checked, and then just pick an environment to place outside - using only one render (and no magic tool struggling or ugly white lines at the edges).