Same as an Abacus.
But every time you need to calculate remaining duration you would need to compute it from remaining volume of work, a hidden calculation some people, like the Owners' rep. want to know if remaining duration was faked or not as an artificial reduction of float. You still need the relationship between resources and their productivity, cheap software do not realize this relationship.
Beware that productivity or production rates cannot be applied blindly, you can determine duration of an activity for a given crew but by increasing the crew size you do not necessarily increase production in linear proportion. With Spider Project the use of multi-resources with various composition and production rates will allow you to model different possible crews with true productivity of their own, if using skills the software will allocate the best possible crew deppending on the avilability of the individual resources that makes up the milti-resource.
Also when modeling the effect of seasons on productivity the software must know how climate reduced or increased productivity and make the adequate adjustments in activity duration or even crew composition as an activity that is delayed from summer to winter might be required to use some additional equipment and work a reduced shift.
Believe me there are many other reasons why you need the software to be able to make the computations by itself.
Best regards,
Rafael