Why is bulk density an input?
Gravel, crushed stone, soil and mulch do not share one universal weight per cubic yard. Moisture, particle size, compaction and material type all change the delivered weight, so the supplier's density should replace the planning default.
How should I use the result?
Use the volume-with-waste figure as a first ordering estimate. Confirm compaction, delivery minimums, material grade and the final quantity with the supplier or project team before purchase.