The order have always been mining for materials that they find to be essential for their needs. Fortunately a lot of essential materials that are found on planet Earth, are part of the many sources of energy and nutrition that the order need. The kind of food that the order consume is completely different from human food. The order are capable of eating human food to conceal their true identity, but they get no nourishment from it.
There are many important resources that the order have been able to find on Earth, but 99.9% of the materials mined from Earth, can also be found on other planets in the universe. There are some that are light years away from Earth, so this makes it convenient for the order to use Earth for those resources. The order never takes more from the Earth than it can reproduce.