Density in additively made parts refers to the porosity of a part and is usually an issue with laser sintered additive technology. (see what is porosity)
Some parts require as high a density as possible in application. Parts that must operate under cyclic loading must be strong (dense) enough so as not to fatigue or fail under load. Several additive technologies can deliver densities of 98% and higher. (CNC machining and injection molding, however, can deliver parts of 100% density. The reason a part may be additively made for density rather than machined or molded would be primarily because of cost and geometric complexity.)
In an additive build, usually during a laser sintering process, the layer-by-layer melting can be incomplete before subsequent layers are added on. This incomplete melting results in holes, cavities, or pores. Those areas of the part with the pores could more easily fail under load than other areas of the part. Depending on the part’s application, such failure could be minor or catastrophic.
Some of the factors contributing to porosity include powder particle size, build chamber temperature, and laser speed and temperature.
Additive manufacturing vendors are working on ways to control porosity and density in those parts that require it.