What is the origin of the tag property in UI toolkits?

I've seen the tag property in all UI frameworks I worked with: UIKit, Forms, OpenStep, even Delphi 6. It's especially baffling considering how useless and bad style it is. How far does it go? Was it in the original Macintosh Toolbox or Xerox's framework? Why was it put there in the first place, and why is it still here after all these years?