Starting in Windows 2000 and continuing through Windows Vista, you could drag a folder out of Explorer and slam it into the edge of the screen. When you let go, it docked itself to that edge of the screen like a toolbar. A customer noticed that this stopped working in Windows 7 and asked, "Was this feature dropped in Windows 7, and is there a way to turn it back on?"
Yes, the feature was dropped in Windows 7, and there is no way to turn it back on because the code to implement it was deleted from the product. (Well, okay, you could "turn it back on" by working with your support representative to file a Design Change Request with the Windows Sustained Engineering team and asking them to restore the code. But they'll probably cackle with glee as they click REQUEST DENIED. They will also probably add a buzzing sound just for extra oomph.)
The introduction of this feature took place further back in history than I have permission to access the Windows source code history database, so I can't explain how it was introduced, but I can guess, and then the person who removed the feature confirmed that my guess was correct.
First of all, very few people were actually using the feature. And of the people who activated it, most of them did so by mistake and couldn't figure out how to undo it. (Sound familiar?) The feature was creating far more trouble than benefit, and by that calculation alone, it was a strong candidate for removal. Furthermore, the design team was interested in a new way to use the edges of the screen. Nobody could figure out how the docking feature actually got added. We strongly suspect that it was another rogue feature added by a specific developer who had a history of slipping in rogue features.