Allows for better preview of the item, as well as drag-and-drop (shift-click?) to set the item. Also prevents invalid strings from being entered.

Relevant info (from this Wowpedia article):

Blizzard FrameXML code:
https://www.townlong-yak.com/framexml/live

FrameXML and artwork:
https://github.com/Gethe/wow-ui-source
https://github.com/Gethe/wow-ui-textures

This is actually taken care of in v2.0 (the rewrite of #2). All the behavior is merely emulated and there is no "special" way to do it. The only currently missing functionality is described by #35.