The benefits of using genetically modified organisms (GMO) for food are generally economic (with some possible health and environmental benefits). The costs of using GMO crops and animals are (if they exist) more environmental and health related.
GMO foods can be very beneficial economically. These foods can be raised more cheaply and in greater quantities than regular foods. This allows more people to have access to more foods at lower prices. This could be very beneficial, particularly in places where food is scarce and expensive. In addition, GSO foods could be environmentally beneficial. They could be engineered in such ways as to need fewer chemicals, for example. They could also be healthier. It could be possible to put genes in GMOs to give them healthier properties, such as putting proteins into tomatoes.
So far, there is no hard scientific evidence that GMO foods are harmful. If they are, however, it is for environmental and health reasons. There are fears that GMO plants will breed with wild plants. This could have unforeseen environmental impacts. There are also fears that GMO foods are dangerous to humans simply because they are not strictly natural.