The answer is that it opens up more bands for transmit.
The reason for this is because YAESU does not make different radios for different parts of the world and since each part of the world has it's own bands for amateur radio operation - the only way to keep people from using the radio as a all band transmit / receive is to block out certain segments of the bands by adding the diodes.

The radio will operate on most any public service band - such as fire, police, emergency - Ambulance and will even transmit on the 800 - 900 Mhz cell frequencies.

The problem with opening up those bands is that once you open them you have to be real careful - to know where you are at when you are transmitting or you might find yourself in a lot of trouble when you transmit on a band for which you are not permitted to be on.

I can hear aircraft bands on my scanner on the 155 - 156 mhz frequencies.
Not to say that it might not be a 3rd order harmonic's of the aircraft bands - just to say that I hear a lot of activity on say 155.730 mhz pertaining to aircraft.

It doesn't make much of a difference - because the local police uses CTCSS to block out any signals which are not encoded with a PL.
But if the person had the PL - it could get them into a lot of trouble.