This is a post by Dave Martin on the PTC site.
Imagine, in these days of working from home, you decide to have a custom desk made. You find a local carpenter and tell him what you want: the type of wood, the dimensions, how many drawers, and so on.
Another customer contacts that same carpenter a month later about a desk, but all the specifications are different for the second desk. In 2D CAD — drawings — the carpenter would redo all their work. However, thanks to the Save As feature in 3D CAD, the carpenter could reuse the first design and save himself from reinventing the wheel (or desk in this case).
This situation is common in the design world. This is a manufacturing strategy known as engineer-to-order. New models and drawings are generated for each custom order. And it can generate a lot of extra work.