Artwork and templates are often used together to create a wrap design that will fit properly fit and land exactly on the vehicle as planned.
If you doing a wrap—such as all camo on a truck, for example—and you have the truck in your possession you can simply measure the truck and then create correct scale panels that will fit those areas. The print/laminate and install is easily done. However, if it's a commercial wrap, you'll need to make sure your logos, text and graphics all land in the exact right location on the vehicle. In a case like that, you'll need an accurate template ahead of time to do the layout, get approval, then output the wrap—even without the vehicle present—but you'll have to make sure things land in the right place so the finished job looks like the layout the customer approved because if it's not like the layout, you may be looking at a complete re-do.
Here are two examples of designing artwork and fitting it to a template to fit a vehicle that was in another state at the time of creation. The finished artwork was approved by the customer, then the artwork was sent via Dropbox to the wrap shop for output and installation. The result was a perfect fit; everything landed right where it was supposed to land and the customer was elated in both cases.
If you are doing graphics or a partial wrap, the artwork is primarily the most important, but a close second is, "Does it fit the vehicle as planned?" If you have the vehicle in your possession you can simply measure the area and scale the artwork accordingly and output. However, the job is easier sold if you show a mock-up on a template first, so the customer knows exactly what he's getting ahead of time. And as before, if this is a commercial job that's going to incorporate a logo, text, phone number, etc., you should be using a template so everything lands where it's supposed to land. Using a template helps tremendously and is like insurance that what you initially sold is what you are going to deliver.
This job entailed custom-created artwork by Aurora Graphics to fit a high-end race car. The customer gave Aurora his vision of what the finished wrap would look like, and we took it from there. A few revisions later, we had the completed design ready and approved. The artwork was fit to templates and the customer once again approved the look. We Dropboxed the files to New Orleans Auto Wraps, who then output the panels to their printer, outgassed the prints, then laminated them. Next, the customer dropped off the car and the wrap was installed. The fit was perfect and the customer was elated as the look was a dead-on match for the mock-up. Long story short, it was a high-five job all the way around.