When making homemade ice cream using table salt to lower the freezing point of water why does it matter that has more chemicals than rock salt?

It's actually the other way around, commercially available rock salt is usually 98-99% pure sodium chloride, while table salt is usually around 97-98% pure, with additional chemicals like iodine and anticaking agents added to it. So rock salt is better for making ice cream if you want to minimize any additional chemicals in your food.