Well, because if one is good two must be twice as good! Actually, we can do even better than that by stacking overdrives and distortions.
When using two distortion pedals on my pedal board I usually set up one as a low gain sound and the other as a medium gain sound. I can then combine the two for more gain.
A couple of things to remember.
There really is no right or wrong way to stack Distortion or Overdrive pedal so use your imagination!