OK, I'm going to bite here, and maybe a few of us can learn something... I'll do my best to put things in layman's terms.
For DC, Power = Voltage * Current. Period. Unless you have a crap ton of harmonics (better how you don't in your computer PSU!).
Computer components want a STEADY voltage- they will draw more or less current depending on how much power they want at a given moment. Under computational loads, they need more power, they draw more current. Voltage is constant, current changes depending on power needs. Having more current rating on a PSU just means it can produce more power.
You can't, as far as I know, buy a PSU with more than 18A per 12V rail rated, at least not if you want to have Intel give it their stamp of approval. You also can't get a UL (safety organization) certification if you put more than 20A on a 12V line, and no UL cert means pretty much no one in the US is going to sell it.
Back in the bad old days of yore, the circuits used to produce voltages for computers were not very "smart", or efficient. So, if you wanted better performance, you just put in a bigger PSU, and if you didn't get close to the rating, you were good, you got cleaner voltages because the system wasn't operating near its limit, and components lasted longer because they weren't being stressed near their capacity.
Enter the 21st century. Power converters get "smarter", they use more active devices, allowing for more precise control of the circuit function. This makes it possible to design a circuit that can operate at rated power like a champ. Why? Because it's actually being controlled, instead of just running free. These PSUs are also much more efficient, because they control more closely the flow of energy in the circuit, and are able to minimize waste.
The end conclusion for a system builder, is that a more modern, more efficient PSU can be pused closer to its ratings without detriment to system performance. In fact, some of these circuits actually run BETTER (peak efficiency, etc) near rated load! So, you don't have to put in twice your peak power draw anymore. Of course, if you want to anyway, they are more than happy to sell you a more expensive PSU...