There are numerous situations where fit to serve may need you to be fitter than fit to do your primary duty.
In the RN the most obvious was fighting a fire/damage control. No 999 on the briny.
For aircrew I'd suggest that E&E in a warzone needs pretty high levels of fitness. - fat/unfit and in the bag with electrodes on your b@lls, or fit and picked up by CSAR -
