In ADSP-BF609 EZ-Board SCHEMATIC (Board number: A0269-2011, rev: 1.0), three regulators are used (3.3V, 1.8V and 1.25V) in sheet 16:
1. We do not understand what is the use of D1, D3 and D4 diodes and they look be overrated (10-12V for 3.3V, 1.8V, 1.25V).
2. We do not understand what is the role of D6, D7 and D8 diodes.
As Greg mentioned most of the diodes are just protection diodes. D1, D2, D3, D4, D5, D7, D8 , they are not needed for the functionality of the power supplies. I think they have been placed there in case someone accidentally shorts things out while probing the board.
D6 on the other end is definitely necessary for the ADP1864 to operate correctly, without D6 it would not work. This is because the ADP1864 is a asynchronous controller and needs the diode during the off time cycle.
you may want to see the ADP1864 datasheet