The drug free period is to allow drug to get excreted. many metal drgus have long t 1/2. (t half peiod). It means they stay long peiod (days or weeks even months) in our body. Continuous administration of such drug slead to cumulative toxicity. Thus, we allow them to get excreted before starting next course.
Another point is ; many toxic drugs destroy normal cells (inhibits normal enzymes) of body tisuses. Continuous inhibition/destruction leads to side effect. Thus, while giving such toxic (to normal cell) drugs, we give gap. So, that the normal body cells are allowed to regenerate or replinish. It is practiced in aniticancer drugs in allopthy. Cancer drugs are not given continuously...they are cyclic.