One of the reasons is that the harmonics of the F make a nice resonance with the segah(E) note. The second one, it gives more emphasis to the first
jins of Rast. So when you reach Nawa ( G) there is more of a surprise and feeling to reach a higher point.
It makes Bayati sound very beautiful on some ouds, especially with very low tuning.
On Nahawand, it prevents it to sound too much like a minor scale.
On Saba, it gives the tonic.
This is how I feel it, it has nothing theoretical. |