There's like 12 exhaust clamps in the system if you don't weld anything. They're ugly and they get snagged on stuff. Plus, they pretty much all leak after awhile. Sure, you can run the system without welding it but it's just a cleaner looking system if you do weld it.
Once the system is fit and everything is where it needs to be there's no reason to have the over axle and tail pipe sections be separate. Or there to be three separate pieces from the headers to the mufflers what with the header reducer and then the 1st and 2nd midpipe sections. Welding it so there's a one piece midpipe and one piece over the axle to the tails with connections only at the headers and mufflers makes it a nice clean system with fewer places to leak. And still enough sections to break out if you need to repair something.
