Counter-missionaries protect unsuspecting Jews. Other than a few basics in the goyish testament, you need to know TANACH and how missionaries distort and take things out of context.
Missionaries mostly use the TANACH in their perverse way, since most Jews, even ignorant ones, would just ignore them if they preached their bible.
Every Jew should know the basics of Counter-missionary information.
From a psychological standpoint, people’s desire for something gets stronger when someone is trying to take it away. The knowledge would probably help those in danger of going off the derech.