Materials which conduct electricity and materials which do not conduct electricity

Semiconductors are materials which are able to easily conduct electricity at a level about midway between conductors and insulators. Silicon is currently main material used for the semiconductor components which are essential to electrical circuits.
So why are materials which only partially conduct electricity used?
One might think that it would be better to use materials which more easily conduct electricity for components which conductor large amounts of current such as power semiconductors.

So why are semiconductors used?

If talking just in terms of ease of electrical conductance, metal would be clearly superior to semiconductors.
But what is interesting about semiconductors, is that they can conduct current or not conduct current depending on conditions. Metal cannot be made to not conduct electricity, however semiconductors by their nature can rectify and control this conduction.