Lower Level - Meaning less abstraction of the actual java/c that is run by LibGDX/Box2D. It just feels more java to me, and probably because as they point out it's not an actual engine really, more of a framework. Yet this means more code for you to write generally.
Testing - IMHO the best part of LibGDX is it's cross-platform with PC which makes it amazingly easy to implement, test, debug, implement, test, debug... and so on. Plus, you get to see your game running at actual speed (actually faster probably because it's on PC and not mobile)... closer to the actual end result of the project on a mobile device. The fact that you could sell your game to PC users is a bonus.
Now to the point:
I think if we could make an extension, or even a third party emulator that would allow us to do something similar with regards to testing directly on the PC hardware it would be AMAZING! Allowing developers to see their programming in real-time (without an amazing device on hand) and save them a good amount of time collectively as they test more and more.