Without D63, the FETs may overheat when the battery is low.
The 3.3v regulator always causes a voltage drop, so if the input voltage is less than 3.7 volts, the 3.3v net will start to sag. Since the battery can go as low as 2.6 volts under load, the 3.3v rail can go all the way down to 2.2 volts. Since the CPU actually runs on 1.8v internally, it can keep running, but when this happens, the FETs are in trouble, because the gate voltage isn’t high enough to turn them on all the way. What happens in this case is that the FETs will have higher resistance than they are supposed to have and they may heat up and die, depending on how much power is going through them.
WITH D63, the input voltage to the regulator will be ~5 volts, which means that it can produce 3.3v output even if the battery is low.
Now, some boards have different FETs on them with a gate-source-threshold of 1 volt. Those boards would probably be fine without D63. Although the amp, the SD card or the motion chip might crap out if the voltage goes too low, but at least there wouldn’t be any permanent damage.