Conventional delta wings can struggle with control saturation at high incidence. The flaperons run out of authority in pitch and roll. Foreplanes enable the flaperons to operate around their mid position so you don't run out of control authority at the extremes. They also help with trimming across the Mach range. Foreplane/deltas can also achieve performance targets with lower overall masses, helpful if you believe there's a correlation between mass and costs.
Having said that, PN might be closer to the truth i.e. fashion may have a lot to do with it. Quite a few years ago part way through Typhoon development, an aerodynamicist suggested that a foreplane delta probably wasn't the best concept based on the latest thinking. He mentioned a foreplane delta with a tail was the way to go. If you believe the SU30's odder manoeuvres are the future of air combat, he might be right.
EAP
Last edited by EAP86; 5th Nov 2016 at 10:50.
Reason: Corrected the Sukhoi designation. Thanks RP