Explain why the boiling point and the melting point can be used to help identify substances-?

It's because substances have different boiling point and freezing point. By finding the exact boiling point and freezing point, you can identify a substance.

Keep in mind that a material's melting point is the same as its freezing point. These are just different terms for the same thing, it just depends on whether energy is being removed from a substance (freezing) or if energy is being added to a substance (melting). The same thing also applies to the boiling/condensation point.