Because of the seemingly innocuous combination of half-steps and minor thirds!
It's one of those symmetrical scales that I just love, although I know nature abhors perfect symmetry, and true beauty (like those lovely Japanese gardens) operates on the principle of slight asymmetry. But for composers, symmetry in the context of tonality is very useful when you don't want the pull of a tonal center. It frees you up to think of other ways to pull in the audience.
Most of us flutists know some symmetrical scales:
1) Chromatic = half steps repeated
2) Whole Tone = whole steps repeated
Then you may know, especially if you have studied Jazz:
3) Octatonic (a.k.a. Diminished) = either repeating half step/whole step or whole step/half step
And the subject of this blog entry:
4) Augmented scales = either repeating half step/minor third or minor third/half step
You find these scales in music by Dutilleux, Gaubert and if I'm not mistaken Jolivet. That minor third makes things sound sort of "harmonic minor-ey", pentatonic or bluesy, depending on the context.
But my point is not that they just sound cool, they kick butt because they are seriously challenging to play smoothly! Why?
1) The half steps go naturally quicker than the minor thirds
2) The scales with A#/Bb also have F#/Gb, so you can't use the Bb thumb with good conscience!
Just try them out!
(3 pages, pdf)