To expand on what DQ wrote----hybrids are bred for certain characteristics or multiples thereof.
They are typically bred for disease resistance, insect resistance, shorter seasonal requirements, better overall plant vigor and health, to prevent lodging (plant falling over from wind, rain, other), determinate/indeterminate (yield period), sprawling plant vs. compact one, higher yield from larger fruit or greater production, greater uniformity of production (size, shape, color), as with melons and tomatoes cracking resistance, thickness of skin of fruits, longer storage time (fruit and vegetables not grains), and taste while still maintaining the aforementioned qualities.
Now added into the equation are controversial genetic modifications which may add health benefits such as beta-carotene to the crop with some modifications even adding pharmaceuticals.
Modifications can also insert Bacillus thuringiensis (Bt) into plants which is normally a naturally occurring bacterial disease of insects thus allowing less or no pesticide spraying.
http://www.ext.colostate.edu/PUBS/insect/05556.html
Crops are also modified so that they may be directly sprayed with certain herbicides for weed control while allowing the crop to actively continue to grow. Less or no weed competition means greater yields without or with less foreign matter in the end crop.
Perhaps you can tell I'm a proponent of hybrids and most generally accept my governments acceptance of genetically modified organisms. Enough on that as I'll probably get yelled at and screamed at by those against such.