Why Ferrites are used as a cores for inductors
Permeability and saturation affect maximum field strength, operating current, and energy per pulse.  Higher resistance/separation/layering of materials are used to reduce eddy current losses.  The higher the frequency, the less inductance is necessary, so air core or PCB trace inductors see use in some applications.  The promising sounding but potentially difficult to control resonant cuk converter may be able to operate at significant power levels with PCB trace inductors, even at low frequencies.  If the factors of scale being talked about pan out, physical size of power converters could decrease by an order of magnitude.  There are reasons to use ferrite(apparently high resistivity/permeability and low cost to manufacture), but it is far from the only material used.  Each core material has a viable frequency range and energy density and when the goal is known, as it often is in engineering applications, it becomes as much a question of which type is necessary as of which type is best.