For a variety of reasons. Some crops are genetically modified to be resistant to herbicides — such as Monsanto’s Roundup Ready soybeans — so that it’s easier for farmers to spray fields with weed killer. By contrast, Bt corn is modified with a bacterial gene in order to secrete a poison that kills pests such as rootworm. That can reduce the need for chemical pesticides.
There are other potential uses, too: golden rice has been artificially fortified with beta carotene, to help alleviate vitamin deficiencies in countries like the Philippines. (So far, however, golden rice is still in early phases and has met opposition from protesters.) And many researchers are looking for ways to engineer crops that are resistant to drought.
Genetic engineering isn’t any one thing — it can be used for a variety of purposes. In practice, large biotech companies like Monsanto tend to focus much of their research efforts on traits like herbicide resistance and pest tolerance for major cash crops like corn, soy, cotton, and canola. At the same time, academic researchers, such as UC Davis’s Pamela Ronald, are interested in harnessing genetic techniques to boost sustainable agriculture or address world hunger.