Can it do something that DS4Windows can't? You can pretty much bind any controller button to any key, macro etc. with it already.
The UX / UI is obviously better with this one, of course. But I don't see any new functionality per se.
Hard to say exactly, but if it's the full works then you should be able to do everything DS4Windows can and more.
The biggest draw however will be action sets and activators, especially in games that have native support for actions instead of inputs. You'll be able to do things like configure different bindings when a mouse cursor pops up, set actions based upon gameplay features (Like making CS:GO's buy menu control with the analog sticks, or adding gyro support on a per game basis, or customizing every game in the Atari Vault on per game basis), and generally have a huge degree of customization.
You'll also get any improvements that are made to the Steam Controller too, providing they're not reliant on a specific hardware function.
And you'll be able to share your bindings with others too.