The function of certain focal lengths has changed with the introduction of the APS-C-sized digital sensor. A 30mm lens, once considered fairly wide, becomes equivalent to a standard 50mm view, while a 10mm or 12mm optic, once the place of a fisheye, becomes a standard wideangle.
Although cheaper, lighter lenses are now designed purely for these smaller sensors, professional lenses are still designed for full-frame use. It is rare, however, that both formats are considered, as it is difficult to satisfy both from one lens without overly inflating the cost or sacrificing the quality.
The Canon 8-15mm f/4L Fisheye USM, then, is unique in that, despite being a full-frame lens, the zoom range contains markings for APS-C and APS-H sensors, which correspond to the minimum focal length that can be used without vignetting.
Using this lens on a full-frame camera will provide a circular fisheye at its minimum and a full-frame wideangle image at its maximum, while on an APS-C or APS-H sensor it will produce a partial vignette at its widest point.
With this information it might be concluded that the lens is designed mainly for the full-frame user. After all, the L-series badging is usually for the realm of the professional or EOS 5D user. However, on an APS-C sensor this lens performs a similar function to a 12-24mm optic, which has always been a nice range for landscapes.