What Are The Possible Benefits Of Genetically Modified Crops?

Farmers like them a lot.  The main GM crops on the market facilitate weed control tremendously (the crop itself is immune to a herbicide that kills weeds).  GM crops also offer more flexibility in terms of when herbicides are applied, and they offer a better yield (more crop) so more profit.  The flexibility in timing of herbicide application reduces the need to plough, which also reduces soil erosion and emissions of climage-change gases.  The herbicide developed specifically for this use (glyphosate) is said to be low-toxic (safe enough for people to drink neat) and degrades quickly in the environment.

About 5% of the world's pesticides are required for cotton alone.  Insect-resistant GM cotton requires measurably less insecticide applications than the conventional varieties.  If less insecticide needs to be applied, then biodiversity and numbers of non-target insect species should increase.  

A number of food crops have been developed or are being worked on to increase their nutritional content, especially Vitamin A in rice (so-called "Golden Rice").  This could address persistant dietary deficiencies in parts of the developing world.  Anti-obesity and anti-allergy crops are also under development.