Right. It keeps you from designing machined parts that are very difficult to clamp, sheet metal parts that cannot be bent with standard machinery, and similar hard-to-make objects. There are simple things which make a part very expensive. A square inside corner in machined metal is one such. Rounded inside corners are cheap. A CAD program which knows this is a boon to the novice designer.

Autodesk Inventor's sheet metal design tools will let you design things in sheet metal which are not makeable with standard machinery. It will show you the unbent flat form of the part, and allow for the metal thickness at the bends, but there may be no sequence of bending operations that works because the previously bent part gets in the way of the tools. eMachineShop understands the limitations of bending.