In the USAF, average sortie durations increased a lot in the last decade or two, and the sheepskin seat covers really help cut down fatigue. They even reduce minor skin injuries (think bedsores and rashes) by allowing some air to circulate between the seat and the pilot. The reason why sheepskin is used instead of other materials such as foam is because the sheepskin has good compression qualities so during an ejection, the seat doesn't get a running start before pushing the pilot up the rails, like it would if a foam or other air filled cushion was used. Giving the seat even a 1/2 inch head-start before the pilot starts moving can be enough to break the spine or femur just above the knee.
