  The Wide Field Imager (WFI) is one of two instruments for the Advanced
Telescope for High-ENergy Astrophysics (Athena). In this paper we summarise
three of the many key science objectives for the WFI - the formation and growth
of supermassive black holes, non-gravitational heating in clusters of galaxies,
and spin measurements of stellar mass black holes - and describe their
translation into the science requirements and ultimately instrument
requirements. The WFI will be designed to provide excellent point source
sensitivity and grasp for performing wide area surveys, surface brightness
sensitivity, survey power, and absolute temperature and density calibration for
in-depth studies of the outskirts of nearby clusters of galaxies and very good
high-count rate capability, throughput, and low pile-up, paired with very good
spectral resolution, for detailed explorations of bright Galactic compact
objects.
