The Blizzard WoW Interface ToolKit is used for assisting AddOn development, or for those who are curious and want to learn more about WoW internals.
The Blizzard AddOn Interface ToolKit provides extra files and resources relevant to WoW AddOn development. These files are taken from the retail WoW MPQ files found in the WoW install and are bundled seperately so that WoW AddOn developers can look at and reference FrameXML files, Lua files, and other art and resources found in the game itself.
Most files used by WoW and the Blizzard WoW UI components are foundinside MPQ files, which are sort of like .zip files and not easily accessible for reference. Blizzard provides these files as a convenience for AddOn developers and the community in general.
Just as there is no official Blizzard support for WoW AddOn development, there is no official support for the AddOn Toolkit either. However Blizzard has maintained a new AddOn ToolKit and support page for every major version of WoW thus far.