1. What's the Gerber file?

  Gerber is the standard file used by PCB(printed circuit board) industry software to describe the printed circuit board images: copper layers, solder mask, legend, etc. 

2.Why using Gerber file?

  First of all, electrical engineer and PCB engineer have different standpoint on PCB, many parameters may show in your file, and you don't want to show these on PCB, and PCB engineer haven't got you, then he/she just make PCB the same as what files indicate,  finally, your parameter has been exposed on PCB to others.

Secondly also the most important, Gerber file can help protecting your achievement.