Most 3d programs have separate control for bump & displacement because it allows for greater & more subtle control of these attributes.  This way 1 map doesn't have to control two attributes.  

The above pic is a perfect example.  See how the displacement map is causing a smooth cross rise, while the bump map is making the grit?  There is complete control there because the bump can be adjusted without affecting the displacement.

In short, it is for better, easier, & more precise control.