A Segulah Not to be Damaged by Sorcerers:

The Dubno Magid zt'l (Sefer Ohel Yaakov) explains this pasuk with a moshol: A general of an army once heard about a wise man who lived in a faraway land. He heard that this genius had created an ointment that, when smeared on the body, made it impenetrable to arrows or spears. Since this general was in the middle of fighting a war, he wanted this cream very much. He traveled to the wise man's homeland and paid him a fortune for his product. The man smeared it on his body, and he set off to return home. On the way, he bumped into a group of robbers, who began shooting arrows at him. The cream did its job and he was not hurt at all. When the thieves saw that they couldn't harm him, they started to flee. However, the general called to them in a gentle voice and said, "Don't run away. I won't hurt you." They came to him and he gave them food and drink. They asked him why he was being nice to them after they attacked him, and he answered, "This trip cost me a lot of money. I had to pay a lot for the ointment, as well as for travel expenses. I knew that when I got home, people would ask me why I spent so much on an unproven product that I didn't really know worked. I was too scared to test it myself by sticking myself with an arrow or knife. But you have done me a favor by shooting arrows at me and proving that it works. Therefore, I owe you something." Bnei Yisroel had always known that since the time of Yaakov Avinu, they had special protection from sorcery, and that no form of magic could harm them. They knew that Yaakov had lived in the home of his enemy, Lavan, who had attempted to hurt him with sorcery but had been unable to do so. However, they had never tested it on themselves, so they couldn't prove it. Perhaps some of them were lacking in emunah and didn't fully believe it. One everyone saw how Bilaam had been unable to harm them with his powers, however, it was proven to all that they possessed this power. Thus, Balak told Bilaam: Look what you did? Not only didn't you harm them, you also did them a favor by proving that they cannot be harmed by sorcery! He was saying that it was already known that evil magic cannot hurt Yaakov's descendants but now, you have revealed for all to see that Hashem continues to protect them.