Alloys are useful (if this is what the question is getting at) because mixing elements of the periodic table alters their colligative properties.
i.e. mixing two metals could change substance's melting point, allowing it to more easily resist high temperatures. Also, mixing metals can change the way they bind together, and create an altogether stronger material than you can find using any single metal alone.They can improve a metal's properties
Vibrations in molecules
hydrogen bonding
Molecular solids
Dirt is a heterogeneous mixture.