I don't know about Monsanto's product specifically, but a lot of genetic modifications aim for larger yields; greater inherent resistance to pests and/or disease; greater resistance to extremes of temperature and/or moisture; and greater concentrations of certain nutrients, among other things. There's a lot more to it than just pesticides.

And pesticides aren't exactly an optional luxury, either. People can die if an entire region's crop is eaten by locusts or aphids or caterpillars or whatever.