Most new parts change several times as they evolve from a conceptual drawing to a physical object in the real world. By the time a new part is ready for prototyping, you’ve made drawings, calculations, and prepared extensive documentation (and if you’re working within the automotive industry, you’ve also spent countless hours on the Production Part Approval Process, or PPAP). And now it’s time for functional testing or evaluating fit within an assembly.
Most designers of stamped parts don’t need to be convinced of the value of specifying tolerances as a regular practice. This example in Machine Design illustrates why tolerancing is always a good idea. “A machine shop that sees an untoleranced diameter, without knowing the design intent, may apply a standard tolerance for three-decimal-place untoleranced dimensions, ±0.005 in. [which] may result in interference, where the hole is smaller than the shaft diameter, which prevents the parts from sliding together … if too large of an interference exists, it will degrade performance.” As the article notes, the end result is usually extra time and money to rework the parts.