General purpose diode between 2 power rails
It is there to protect IC's that require the 12V at all times that the 5V is present (possibly older memory ICs). If the 12V drops faster than the 5V supply on power down, you could have 5V still when the 12V bias is gone. This insures that if the 12V drops faster, it will not 'over-run' the 5V as they approach 0V. Some chips can be destroyed if the 12V is not turned on first or simultaneously with the 5V, and at power down, the 12V must stay above the 5V supply at all times. I assume the difference due to the .7V diode loss will not hurt the ICs.