Seems a simple question, but I can't find it in the docs anywhere. A number of functionalities are noted as being Editor Classes, which means that one has to import a secondary namespace (in C#) to use them.
Okay, that's no problem -- but what other drawbacks are there? Are these editor classes slower? Only available when run through the Unity editor, and not as a standalone app? In other words, what is the practical reason for separating these out as a whole separate branch of classes? Or is this just an organization thing?
You guessed it, the Editor classes are exclusively used for manipulating the editor, itself, and not used in the published games.
You cannot edit the source of the editor, but they provided these editor classes to allow you to make direct manipulations to speed up your workflow and to support advanced features.
You'll notice on the Unity Extensions page, there are plug-ins which support easily creating explosions, a more advanced terrain editor, a full locomotion system and more. These enhancements update the GUI and script actions that would be tedious to do by hand. These plug-ins use the Editor classes extensively.