The inner holes makes the bar stiffer, the outer holes makes it softer (more or less flex in the bar). These changes makes that end of the car softer or stiffer. If you make the rear bar stiffer, the rear end of the car will be stiffer, and therefore give you more oversteer. A stiff front end will give you understeer. So in some ways a softer car will give you more traction, BUT in higspeed corners you want a stiffer car to make it stable. In a race car you also need more stiffness to produce enough heat into the tires. So back to square one. You need to adjust your car to get the optimal mix/balance of what you need AND that fits your driving style. This is the same reason a racecar on slick tires can be pretty stiff so you really can use downforce, tires etc. But in rain you need so loosen everything up, softer springs, softer sway bar setting (or disconnect them) etc becuase a dry weather set up is useless in rain with lower speed, no grip and wet(cold tires). That's why street cars can be really quick on track days if it rains compared to Cup/RS cars.
It's more complex than this, but this is the simple version!

Thomas

 

Thanks for the great explanation Thomas.