The purpose this serves is to slander and frame the alien races for doing it, so that humans will fear the alien races instead of trusting them and learning things from them..[1]

The alien races do not mutilate animals.

The alien races do not leave evidence.

I believe it was the government mutilating cattle, to frame the aliens and scare the people.

There is an obvious contradiction between the government hiding evidence of alien life, and displaying dead cattle.