Actually most people think that the 12V rail is required for supplying power to the Graphics card which is actually not true. Actually the 12V rail delivers the current to most of the components and also minimize the current loss
Computer peripherals like CPU, mobo needs high amount current in low Voltage. Normally the dencer components like CPU, GPU need low voltage power.
Now you guys know that
I= V / R and P= V X I
I=Current flowing to the circuit
V=Potential difference
R= Resistance of the circuit.
P= Power
Now if you reduce V then for supplying same amount of power, I needs to be increased.
Initially the 5 V rail was used to supply the 50-60% current to the circuit but that leads to high power loss as for maintaining low voltage with high amount current, the resistance needs to be increased which leads to significant power loss. For minimizing this power loss the concept of 12V rail has came to the picture. A good PSU should deliver its 85 to 90% power through its 12V rail and the power output in 12V rail is more important than the total power output.
Now if a single 12V rail is used to deliver the 85-90% power of the total power then this wires need to be of very very high quality with minimal resistance for suppressing heat generation, power loss and most importantly damage to the wire. Thats why some of the PSU use multiple 12V rail.
Typically, a power supply will guarantee at least 17 A at 12 V by having a current limit of 18.5 A, plus or minus 8%. Thus, it is guaranteed to supply at least 17 A, and guaranteed to cut off before 20 A.
These groups are the so-called "multiple power supply rails". They are not fully independent; they are all connected to a single high-current 12 V source inside the power supply, but have separate current limit circuitry. The current limit groups are documented so the user can avoid placing too many high-current loads in the same group. Here the total power that can be delivered through the 12V rails are not specific to individual rails but actually to assigned to the whole group.
But most of the premium PSU makers actually use single 12V rail to omit the extra complexity and circuit limit circuitry. They actually use very very high quality components to deliver the whole power through the single 12V rail.
So dual 12V rail with 18A each does not mean that it can provide (2X18 X 12) = 432W power. It can only provide the amount of power that has been specified by the manufacturer to the whole group. So if a PSU has two 12V rail with 17A each and the total 12V rail has a rating of 350W then each of the wires limit is also 350VA, not 408VA.
Although dual or multiple 12V rails look more promising than their single rail counterpart, there are some problems. It is because assignment of connectors to rails is done at manufacturing time, and it is not always possible to move a given load to a different rail.