Some people may think or refer to think as ‘tracing’. As a designer, we refer to them as underlays. Underlays are extremely helpful guides when it comes to design. With underlays, you are able to make professional looking sketches in minutes. As, for example, the shape of an interior does not happen to change in size depending on the category of car you are sketching for, you can use an underlay to fine tune a rough idea. For example, here is a quick sketch of an interior I did.
Rough sketch of an interior
As you can see, it literally is just a rough sketch, but if I use an underlay, like the one shown below, I am able to translate the rough sketch into a professional looking, more fine tuned sketch in no time at all. You may even think of new ideas when fine tuning the sketch, so your rough sketch idea may not turn out to be exactly like the fine tuned sketch. As you can see below, I was working on one earlier. Except the design has changed slightly using the underlay.
Using an Audi Interior underlay
So, as you can see, using an underlay is extremely useful in putting out a number of good looking sketches in no time at all. This particular fine tuned sketch is unfinished but I think you understand the point. This technique is useful when expressing your ideas when you are sticking to a certain time limit.