Back half necessary?

I just picked up a 1998 rcsb 1500 2wd that I'm going to be dropping my p-pump 24v in. Will see some street miles, but mostly track. I plan on going with a 4 link rear, double adjustable qa1s, narrowrd 9inch etc... Where my big question lies is is it necessary to back half. I pulled a measurment from the outer edge of the tire to frame rail and got 18 inches. So with cutting the wheel wells out and putting bigger tubs in, is there something I'm missing as to why people still go and back half these trucks?

Thanks, Mark.