One benefit of many GMO crops is that they allow farmers to use much less in the way of pesticides and fertilizers than they otherwise would have.

For example, BT Cotton kills cotton bollworms on its own, so you don't have to spray it with additional pesticides that would kill lots of other bugs (such as aphids) and might potentially cause harm elsewhere in the food chain. (BT Cotton and BT Corn both reduce insecticide use by about half.)

http://www.theguardian.com/environment/2012/jun/13/gm-crops-...

http://www.pbs.org/wgbh/nova/next/nature/fewer-pesticides-fa...