What is the purpose of the stand off on a wall wart?
One extremely good reason for the standoff is illustrated by this plug-in air freshener. Without the standoff it would be impossible to insert the device into the socket as the switch would get in the way:



Other reasons - as suggested in the comments - include pure cosmetic appearance, ventilation, safety, ease of grasping and, doubtless, others. In some countries, part of the standoff may fit into a recess in the wall outlet to provide protection from accidental contact and make it harder for the device to fall out accidentally, and the device may be unusable or unsafe without it.

You need to do a great deal of research into standards and use cases to determine whether your product needs a standoff and, if so, how large. It would be embarrassing to launch a product that a number of people wouldn't be able to use.