Pardon the ignorance, but why are these diodes required?

Robert,

 

When there is a failure on either the left or right bus, the remaining good bus can drive the avionics bus and the essential bus without back feeding to the bad bus.  Switches would greatly complicate things.

 

The Perihelion Schottky power diode is excellent because of the low voltage drop at reasonable power draws. That is that the buses so connected will operate at a slightly lower voltage than the main buses. 

 

I use one to isolate my essential bus from the main bus when the essential bus must be switched directly to the battery.  Also one is used to isolate the electronic ignition and backup battery from the main bus if the backup battery must be called into play (switched as sole power to the EI).  This allows the backup battery to share the charging power from my single alternator during normal operation but be totally isolated if there is a problem.