EPP stands for Expanded PolyPropylene and resembles Expanded PolyStyrene (EPS). EPS is best known as the widely used white foam packaging material. EPP is usually black and known for it's use in bicycle helmets. Compared to EPS, EPP is much tougher.
Molds are not as expensive compared to injection molding while there are more geometrical possibilties. The material is soft which means undercuts are allowed up to a few degrees. Also the material is forgivingly soft which makes engineering the interfaces more secure. Low pressure seals are possible.