Yes it does both (A/D & D/A)

Two big differences
1. Using Surface, Windows is doing the conversion, instead of an interface designed for the task. (much better convertors)
2. While the headphone out is audio, it is a weak audio signal compared to Line level, so the PA has to try to do more with less.
The PA 'expects' a line level signal and is designed for such.

The difference is very apparent when plugging a headphone out (like a phone or laptop) to my powered monitors vs. compared to a line level signal, like an interface. There's a world of difference that simply turning up the Monitor volume can't fix.

I think once you try it you'll notice right away.

I do not work here, but the benefits are still awesome
Make your sound your own!