The sheepskin makes it not as slippy and it also makes it very comfortable.
Back in the days before a grateful nation put up a nine-foot tall life-size statue of me in front of my old skool (take that, Miss Pomeroy!), when I was dodging the previous government's assassins, I used to leave stuff behind. I'd literally put the phone down and say to my wife, "A couple of men with South African diplomatic passports have just broken my publisher's arm for refusing to give them my address. They're on their way here now to kill me. Put the baby and some nappies in the Volvo, we're off to France. Leave everything but the baby." One of few things that I regret leaving behind was a large sheepskin that I used to sit on in Africa, South America, Australia, the UK, France, Italy, Germany, Austria, Switzerland, California and Alaska, cool in summer, warm in winter, and, as you say, very comfortable.
My favorite Herman Miller Mirra chair is comfortable even after 16 hours, and has long since earned its cost in fees not paid to physiotherapists to fix a hurting back, but the only thing that would improve it is a large sheepskin. If I could buy a Brooks saddle made woolly side-out of sheepskin, I would. Be hell to keep clean, though.
