I need it to power on computer?Yes, if your GPU requires external power.
Ok thank you because i have a gt 420If you have a graphics card installed that requires external power from the power supply, then it needs to be plugged in. If you do not have a discrete graphics card or you have one that does not have a 6-pin or 6+2pin PCI-E power port(s), then no. It does not need to be plugged in.
-Wolf sends