Clearance for the front steering/axle linkage. If you are lifted at least 2-3" you can get rid of the crush, otherwise the bar will make it's own crush in your new downpipe...

 

That's what they wanted you to think. I've always been suspicious of the routing and the 90* turn it makes so close to the driveline.

My buddy at JeepTech told me the real reason when I questioned him a coupla years ago.. It's there to slow down the exhaust so the catalytic converter heats up more quickly.

Have one made with two 45* bends and it will work, lifted or not.

I've got a stock height MJ and a 3" lifted XJ with identical pipes as described above.