Surely one minor scale is enough! It would certainly make things easier, but in a lot of cases it would not sound as easy on the ear. OK, so we need to use the notes of the major scale in a way that makes the whole thing sound sad or haunting.
Playing the major scale pitches downwards starting on the 6th degree for an octave (the natural or relative minor) does produce a sad sound but has two serious drawbacks. Firstly, the same scale doesn?t sound quite right when going upwards. The reason for this is our second drawback ? the scale does not have a leading-note. This is the note that makes all the difference to the chords at the end of a section or piece (the cadence), when the harmonies move pleasantly to the home or tonic chord by using the familiar sounding 7th note going up a semitone to the 8th note. Since all harmonies (chords) are built using the notes of the scale, without the leading note the cadences just don?t seem to do their job, sounding more like just two unrelated chords. It was decided that this just wasn?t good enough, and the scale must be given a leading note by raising its 7th degree by a semitone, and because we could use this scale going up or down and we built all the harmonies from it, it became known as the Harmonic Minor scale.
So far, so good, but now it was the turn of the vocalists to complain! Singers found it difficult to pitch the gap between the 6th note and the now raised 7th note of the Harmonic Minor scale, this gap now being 3 semitones or a tone and a half. It was decided therefore to adjust the Minor scale so that it was easier to sing but still pleasing to the ear whilst retaining its sad quality. It was also decided that in order to best achieve this, the scale would be made slightly different according to whether the notes were rising or falling. This was the birth of our Melodic Minor scale, named because it was derived from the melody sung, which, compared to the Major scale starting on its 6th degree, had a raised 6th and 7th when ascending but lowered these notes again when falling to revert back to the natural minor scale.
So now we can please everyone. Singers have the Melodic Minor scale that is easy to sing upwards and downwards, and instrumentalists have the Harmonic Minor scale with which to form all our harmonies and chords. As a plus, everyone can also borrow from the other just to be different!