For thousands of years, wisdom teeth have served no purpose.
Prehistoric peoples' diets consisted of raw meats, roots, leaves, and other foraged foods. To cut up these tough foods, as well as catch, dismember, and consume them, strong teeth were required. Having extra teeth was a huge benefit, especially when there was no dentist nearby to keep teeth healthy. People have evolved since then, and we now have a more refined menu. This means that we don't require these extra teeth to eat tough foods and survive.