The Harmonic Minor Scale was developed due to the lack of dominant resolution from the V7 to I in the harmonized Natural Minor scale. In essence, it is an alteration of the Natural Minor scale "for harmonic purposes." Unlike the Natural Minor, the diatonic V of the Harmonic Minor scale is a dominant chord. In other words, you'll find a tritone relationship between the chord's 3rd and 7th degrees.
Harmonic Minor Mode Construction Diagram:
