@pApA^LeGBa Different playstyles.
It's not about using the exact amount of fuel but knowing how long it takes to smelt a whole stack of anything. If you tell your forge to make so-many bullet casings (or even craft anything in the game) it tells you exactly how long that's going to take so you can put in the fuel you need for that specific job. However it doesn't tell you how long it's going to be smelting for - and that's useful information.
I simply didn't know off the top of my head how long 6000 stone took to smelt. I know it's a background task but I have an assembly line mentality, so the time it takes is nice to know so I can leave it cooking, go out do a mission, come back and it's done and the fire went out automatically.
Maybe I played with my brother at first too much, but he told me always to put in no more fuel than you needed - I never asked why, it was just efficient, but there's obviously a heatmap effect to worry about. I share his pet peeve with my newer friends of coming back into the base to find EVERYTHING merrily alight and loaded with a stack of fuel and nothing in the queue. I'd think you'll agree that sticking in five or six jars of murky and then 2400 wood and leaving it running is a waste, even if wood is inexhaustible.
So, really, it's just handy information to have.