Bed ridden patients used to have sheepskins to lay on, they help prevent bed sores. Same principle on the bike, every time you move the piles move so the pressure points change.
IMHO don't get a fleece from a furniture store, they've been cleaned to an almost sterile state, without the natural oils, lanolin, they will retain water. I use mine as pillow, its 5 years old, been caught in too many downpours and doesn't smell. Could do with a good combing though
Signature;- Current bike, 1991 Africa Twin RD04