Why Do Gym's Have Mirrors?

I guess gym's are a microcosm of life. Anyone who knows me will also know that when exercising I sweat like an MP submitting his expenses. Last week a guy at the gym who I am on 'nodding terms' with offered the advice that "sweating like that won't get you anywhere" when I replied that putting in the amount of effort I do leads to sweating and shortness of breath he replied that "You should take it easy mate, less effort for longer". This might have seemed good advice but not from someone who looks about as fit as Jabba the Hutt.
The mirror is a 'Magic Mirror', I've noticed loads of guys (and some ladies) who sit in front of this mirror, literally a foot away, staring at their reflection with a steely glint, willing themselves fit and/or thin.

Still whatever floats your boat, I enjoy the gym, earphones in, music on shuffle and I am lost to the world.