Why do we need to find bulk density of solid waste?

If we didn't have soilds we would be outside because bricks are soilds and so are skicks and if we didn't have bricks or skicks we could not have a house.

That's a dumb response.

We need to determine the Bulk Density of solid waste so we can properly design and implement waste collection, storage, and treatment options.

Bulk Density of Solid Wastes play a very important part in the design of landfills, design of garbage collection routes, design of waste composting facilities, etc.