Why do the Crookes Tubes require high voltage?

The high voltage is used to accelerate these positive and negative ions to either end of the tube; the electrons moving one way and the positive ions the other. As they do so, they gain energy and when high enough they will collide with other gas molecules and ionize them too. Once the electrons (or cathode rays) are produced and start to ionize other molecules, then these, in turn, will go on to produce more positive ions and negative electrons.