Installed the 18" Wheels on the Duster

Why the use of wheel spacers - due to wheel offset or backspacing spec on these wheels?

Yes. If you were set on the stock style brakes for life, you could probably roll with 5.94 backspacing mustang type wheels and not need a spacer in the front or back. In the back it would be tighter to the wheel lip. I may change to the Dr. Diff Cobra brakes at some point which will add some track width.