The purpose of the nonprintable area is to indicate to your customer that they cannot add any images or text to a particular area of the template. A field set as a nonprintable area will never print on the production file or display in the preview and prevents the customer from adding any images or text in that area.
The nonprintable area does not generate a prompt because it is not something the end user has to provide anything for. It is a blank spot in the design. They cannot put text or graphics there at all and for the interactive designer templates, your customer will find that the item they attempt to drag over that area will disappper.
You can assign a graphic file to the area so that you can put an image which makes it obvious to your user that this is a nonprintable area (crosshatching, or the words "NO TEXT ALLOWED HERE" for example). The graphic will not be part of the production file. If no graphic is assigned to that area explaining to your customer that it is a non printable area, it could cause confusion. That is why a graphic file can be used, and is suggested.
A possibly use of this feature would be a mailing label area that you cannot have any images or text added to in order to comply with postal regulations or if you want to display die-cut lines, instructions, etc., but not have them clickable, or print on the final production file.
While this option was added for the Interactive Designer version of eDocBuilder, it can still be an option for Forms Based templates as well.
You can also achieve something similar if you choose to set the field to be an image coming from a graphic file, you can then choose to have the field appear only for proofing but it will not prevent the customer from adding text or images in that particular area if they are using the Interactive Designer version of eDoc and have permission to add images and/or text.