my idea is this one:
if i have two hundred dollars and want to buy 5 kg meat that costs 65.45 per kilogram, i don't have to multiply exactly to know that I don't have enough money: even if the meat costed 60 per kilo, I would not have enough since then it would cost 300 dollars (and more, since i rounded it down). - here I only need to estimate how much I would pay more or less to know if I have enough money.