When a furnace is below a blockentity like a chest, the furnace will grab items from the chest as if the furnace was a hopper, this would be useful with the smoker and the blast furnace as those only accept some items and would allow for a very simple filtering system for smelting things.

i posted this on the minecraft feedback site too, i think it would be a very useful change, it would also be fun to play around with.

To clarify, only containers above the furnace should get affected, you shouldn't be able to drop items onto the furnace like a hopper.

you can make a hopper item filter system for filtering, similar to a simple impulseSV sorter, so how is this different