Push for Ableton Live was a breakthrough product allowing unimagined control over a DAW and delivering a whole heap of fun in the process. Push MKII simply improves on the original concept and takes this even further.
Ableton Live has probably more dedicated control surfaces available for performance than any other DAW but the Push controller is the first actually designed and engineered by Ableton specifically for integration with Live and for lovers of Live (such as myself) it finally adds some much needed creativity and control that is not just aimed at "live" performance.