if you have rwd and want to break traction easily(drifting) make them harder at the back.
if you have rwd or 4wd and want to do good drag launches without wheelspin make them softer at the back, to push more weight on the back wheels.
for a good handling rwd/4wd car have the backs a small bit softer than the front
just my experiences with setups
luca is spot on about fwd