the attrition gives the % to get a reward, so that if you have 10% attrition, then next encounter you have 10% chance to hit a reward
if you have 65% attrition, next encounter you have 65% chance to hit a reward, if you have 100% attrition, next encounter you have 100% chance to hit a reward (possibly 5fps, possibly 200 fps, possibly 50 diamonds, etc
. At least it would give some goods and troops back, fps are not all there is in this world, and makes it more interesting (there is no massive incentive to push attrition high and be very active otherwise)!