According to the search results, Hagrid went to Knockturn Alley to buy Flesh-Eating Slug Repellent in order to protect the cabbage crop at Hogwarts School of Witchcraft and Wizardry. Although some fans have speculated that Hagrid might have been doing something else, such as engaging in dark magic, there is no evidence to support these theories. Therefore, it can be concluded that Hagrid was simply buying the repellent to protect the schools crops.