Nothing in biology makes sense - except in the light of evolution.

Cancer pervades the biosphere.

Cancer is deterministic and effective, not random in its behaviour.

It looks like a pre-programmed response to some sort of stress. Cells come pre-loaded with a cancer subroutine that can be triggered by a variety of insults.

Cancer might be an indespensable corollary to the rise of multicellurality and hasn´t been eliminated because of embryogenesis.

The same genes active in cancer are active during embryogenesis. 

As cancer progresses it goes evolutionary further back.

https://www.youtube.com/watch?v=yoQYh0qPtz8