My computer has a 24 pin power connector. My computer came with a power supply that only had 20 of the 24 pins covered. I upgraded my graphics card and power supply recently. The new power supply had a 20 pin connector and an additional 4 pin connector. I plugged both in to cover the 24 pins in the computer.
What is the advantage of having all 24 pins covered over just 20?
The extra four pins give two extra ground pins and two extra 12 volt supply pins. The extra pins on the motherboard are paralleled with the other existing ground and 12 volt pins, so the motherboard will operate without the four pin plug connected. However the extra pins give extra current carrying capacity (which is particularly important if you have a high power graphics card) giving improved reliability over the longer term. Failure to use the four pin connector can result in the 20 pin connector burning up.