1) When calculating in inches asphalt weighs 12 lbs/sqft, exactly. That way you can just multiply by 12 and not touch the calculator.
2) When estimating quantities asphalt weighs 150 pcf, exactly. Because you don't want to come up short, even by 4%.
False precision breeds the lie that engineering is "exact".
I agree with what the others wrote, but couldn't help adding my two-bits.