The Cardinals won the first game of the series in St. Louis but dropped two in a row. So the pivital clout saved St. Louis from falling behind three games to one and instead evened it at two games each. Ultimately, the blow decided the World Series because the Cardinals ended up winning it in the seventh and final game.
Boyer only hit .222 in the series. But two of his five hits were homers. In videos of the grand slam, if you look closely, you can see Boyer pat the Yankees third baseman on the behind as he rounds the final corner toward the plate. The New York third sacker is his brother, Clete.