Having eight notes enables the primary chord tones to continuously fall on the on-beats when the scale is played sequentially.
Chord tones on on-beats are characteristic of all strong melodies throughout musical history.
The bebop dominant scale is derived from the Mixolydian mode and has a chromatic passing note added in between the flatted 7th (♭7) and the tonic.
They are also included, with the exception of the Dorian bebop scale, in Roni Ben-Hur's book Talk Jazz: A Comprehensive Collection of Bebop Studies, which is derived from the work of Barry Harris.
Ben-Hur further elaborates on the concept of placing additional chromatic passing tones between other notes in the scales.
The seventh flat 5 diminished scale (which is identical to Messiaen's sixth mode of limited transposition) is derived from the whole tone scale, with an added fourth and a natural seventh degree.