Genetic engineering consists in cutting out a piece of DNA from one species and inserting it into another species. By doing this one can overcome that block and it is possible to breed species with characteristics untypical for that organism which might present an advantage.
A good example is genetically modified
crops:
About half of the American soybean crop planted in 1999 carries a gene that
makes it resistant to a herbicide used to control weeds. About a quarter of
U.S. corn planted in 1999 contains a gene that produces a protein toxic to certain
caterpillars.
The idea behind the modifications is that one does not need herbicides anymore for insect control, which saves money, is more ecological and does not destroy the crop. Moreover the plants will not carry toxic substances, affecting human health.