Dhiwakar Ravikumar's answer is correct in that the subkey contains a list of font files and that Windows won't see font files without an entry there. After some experimentation, however, I discovered that the name of each entry doesn't matter at all, at least for modern programs. (It's conceivable that older programs rely on it.)
Just putting a font file in \Windows\Fonts
won't actually make it used. When you copy fonts there with Windows Explorer, it helps you by registering the file, but if you just put the file there with a command prompt, nothing special will happen and the font won't get registered.
Windows gets the font style names from the font files, so you can party on these values' names without causing any trouble. Font families are also defined by the font files (source), so the "Bold" and "Italic" and whatnot in entry names doesn't matter. If you change the data to point to a non-existent file, however, Windows won't see the font as valid and so won't tell programs about it. Other fonts in the family will still be loaded, though.
In short, this key is just a list of files for Windows to investigate.