Because initially I started working on programming with an arduino UNO, but my arduino UNO got screwed up or stopped working when I removed the microcontroller, trying to upload the programming to a microcontroller.
So I decided to use a MEGA because it was the one I had on hand at the time. Besides that it is just to test that the programming runs well, after that I convert the Mega to ISP and load the sketch to my ATmega328 microcontrollers.