Knee act as a junction which drives your whole leg towards the direction you want to run. So it’s very important to use your knee if you want to drive force from your quads and hamstrings.
This is the basic of running and it stills work because biomechanically it makes human being more efficient to run faster.
The core is the power generator of your body it has many benefits. It keeps your injury-free and makes power transfer much more efficient and reliable for driving your whole body forward.