Teaching major and minor
Natural minor scales clearly illustrate how a minor scale is related to its relative major, how different modes can be derived from one diatonic system by taking the same key signature and starting on different degrees.
This is very easily demonstrated on the piano, where you can physically see the tones and semitones in a way that is more difficult on a wind or string instrument. In fact for pianists this is particularly straightforward, as the natural minors starting on A, E and D all have the same fingering as C major.
If developing an understanding of the natural minor is omitted from teaching, many students fail to properly grasp this basic relationship between the major and the relative minor. This can impact the depth of understanding of the harmonic and melodic minors and why some of the sharps or flats are placed in the key signature and others as accidentals. The question of why the G# in A minor is not indicated in the key signature is often not addressed with any conviction.
By introducing the natural minor in the lower grades, students can best begin to hear and appreciate how scalic patterns really work, rather than simply learning the harmonic and melodic minor scales kinaesthetically by rote as exercises, without fully appreciating or understanding the way they function theoretically and musically.