your normal PSU in your computer takes in AC power from the wall and converts it into dc power which your motherboard and other parts need. Now a sproggy and a opus take the 12v dc power from your car and regulate it and split it into the type of 12v and 5v power your motherboard needs. WHy is that better.. it doesn't cause noise int he system and its quiet. There are many other advantages though.
With an inverter you are taking the 12v power from your car changing it into AC power then it goes into your PSU and switches it back to DC. That can add noise in the speakers and heat. The reason people do it that way is to save some money.
If you use a dc-dc PSU you don't need a normal PC power supply