I'm showing my age here, but one of the stories is it dates back to live axle days and changing the front bar stopped the back of the car from beating the front of the car through a corner.
In most cars with strut suspensions autocrossing, changing the front bar tends to help more than changing the rear. Restating what has probably been posted before, a bigger front bar on a strut car helps reduce some of the roll induced camber loss up front and helps in the transitional response for slaloms.

 

by limiting rear traction, because the rear can't role propperly and transfer the weight. it is better to increase the traction and one end than loose it fron the other!!

 

But honestly, do they really need a good reason?

From what I recall, you can't put in an aftermarket BOV or Intake otherwise it moves you up a class - even though most people are saying neither really does anything for performance.

(Sorry - venting some excess steam.)

 

you can also down size the front the sway bar and induce more oversteer in a car. Its the differences in stiffness front to rear that control the rotation charateristics

 

Try disconnecting your front bar before buying a smaller one to see if it moves you in the proper direction (less understeer/more oversteer). Although the traditional race car tuning says stiffen the end you want to slide first (or soften the opposite end), with a relatively softly sprung street car it is not that clear cut. Many people have reported a reduction in understear by going to a stiffer front bar, likely because the reduced roll gives a better mid corner camber.