These files are used to create the "resources.swf" file in the editor's folder.

This contains a standard set of character and map graphics, as well as sound effects, which can be accessed by the story editor and the story player.
To see which files are readily available when you use the editor, look in the "resources" folder. When you want to use these files, just type their file names. You don't even have to include the names of the folders they're in.

For those familiar with programs like RPG Maker, resources.swf is sort of like the RTP in that program.