it means that each hopper in the chain only needs to fill with 1 stack of 64 coal or ore instead of 5 before the next furnace gets coal or ore, and makes smelting the ore much faster
if you fill a hopper with a non-stackable item eg a sword except for one slot then it makes a system like the one i posted in MC-26729 far more efficient provided that the furnaces only accept smeltable or burnable items...
it means that each hopper in the chain only needs to fill with 1 stack of 64 coal or ore instead of 5 before the next furnace gets coal or ore, and makes smelting the ore much faster