Because pulling heavy trailers is much easier with a dually. And because it can carry loads far beyond the load capacity of a non-dually pickup truck.
Few people buy a 3/4 ton truck just because they like the style, most of the ones I see are either work trucks which oftentimes carry heavy loads of wood or steel depending on the profession of their owner - or they pull heavy trailers, containing everything from cattle to boats.
I have a Dodge RAM 1500, which sees very few miles outside my trips with a heavy trailer in tow. Would I want to drive it as an everyday vehicle? No. Would I want to pull my trailer with a Chevy Impala? Not in a million years.
Chris