The purpose of proppant is to provide a high conductivity pathway for hydrocarbons

The purpose of proppant is to provide a high conductivity pathway for hydrocarbons to flow from the reservoir to the well.  Proppant does this by preventing the fracture from closing due to the stress of the surrounding rock. The deeper a fracture, the more pressure there is in the surrounding rock trying to close the fracture.  If one where to use a low grade proppant such as brady or white sand at depths of 10,000+ ft as in the Bakken, the confining stress of the formation would crush the sand and turn it to powder and it would no longer have conductive properties to allow fluid flow.  By pumping a high density ceramic in a frac job (sand being the lowest strength - generally, resin coated silica sand being the intermediate strength, and ceramic and sintered bauxite proppants being the highest strength) a propped fracture will allow fluid flow even at extreme closure pressures.