Density, mass, and volume are three sides of the same relationship - know any two and the third follows. A plain unit converter can turn kg into lb, but it can't tell you what 200 litres of water, or half a cubic metre of concrete, actually weighs. This can.
Density is the relationship that connects how much space something takes up to how much it weighs - and it's genuinely different for every material, which is exactly why a generic unit converter can't help here. A litre of water weighs one kilogram. A litre of concrete weighs about 2.4 kilograms. Same volume, very different weight, because the material is different. Pick any two of density, mass, or volume and the third is determined - this solves for whichever one you're missing.
The material presets are genuine reference values, not guarantees - real density varies with moisture, temperature, compaction, and exact composition. Topsoil especially can weigh 20% more when wet than dry. Treat these as a solid starting point for planning, and use the exact figure on your specific material's spec sheet when precision actually matters (structural work, shipping weight limits).