The +5VSB is essential to supporting the "turn on sequence" for the
computer. Without +5VSB, the computer won't turn on. The +5VSB powers
chips on your motherboard, and one of those chips controls a signal
called PS_ON#. PS_ON# is pulled up with a resistor to logic 1, and
when the chip on your board grounds the PS_ON# signal, the power supply
then turns on the rest of the supply voltages +3.3V, +5V (main),
+12V, -5V, -12V.

logic 0
ps_on is a active low