why we provide negative DC to some wire in smps in computer?

Question

for example, blue wire in smps gives -12 volts

Answer 1

-12 Volts would most likely be used by some analog comparators (similar to Op Amps). The compartors are used to sense the main voltage for the logic in the computer, or to sense the current across a sense resistor, to provide feedback to the main regulator.

.