The voltage readouts allow you to monitor what your PSU is outputting. For example the 12v output should be around 12v ... if it was 11v you'd most likely have a problem.
CPU/RAM voltage adjustment screens are mainly for overclocking purposes. When sometimes an overclock is not stable because enough power cannot be drawn at standard voltage, so boosting voltage can help the overclock become stable. The drawback is that extra voltage equals extra resistance equals extra heat. And heat, is bad.