3.14Why raise the sixth too?

Remember harmonic minor had:

F → G♯

That augmented second is relatively awkward melodically.

If we raise F:

F → F♯

then the ascent becomes smoother:

E → F♯ → G♯ → A

all whole-step / half-step relationships.

So the historical idea is roughly:

• raise 7 for harmonic pull
• raise 6 when ascending to make the melodic line smoother