Torch height control isnt just for large machines or under water cutting. It is there to keep the correct torch height so that the correct part of the plasma arc is cutting the material and giving the straightest possible cut.
Speed is important to plasma cutting. Travelling too fast will give apoor quality cut and you can easily lose the arc transfer or end up goughing the plate rather than cutting it. Travelling too slow will also give poor quality cut and also a wider kerf.