Don't use this function on a non-X11 platform, because the XSetWindowBackgroundPixmap function that it calls is not implemented in Tk for Windows, or Tk for MacOSX, in their respective Xlib emulation layers.
It is useful for implementing tiled backgrounds that work with multiple layers of windows (via the special ParentRelative Pixmap identifier). Some widget sets use it for pixmap themes.
It should either be fixed or removed from Tk. The core implementation used by X to implement the current XSetWindowBackgroundPixmap is in: xsrc/xfree/xc/programs/Xserver/dix/window.c