What's all this "rails" nonsense on a computer power supply?
Allow me to quote Wikipedia: Power supply unit (computer):


  In computer power supplies that have more than one +12V power rail...
  
  
  Multiple 12V power supply rails are separately current limited as a safety feature; they are not generated separately.
  ... the IEC 60950 standard, which requires that no more than 240 volt-amps be present between any two accessible points. Thus, each {12 V} wire must be current-limited to no more than 20 A; ... Unlike a fuse or circuit breaker, these limits reset as soon as the overload is removed.
  Because of the above standards, almost all high-power supplies claim to implement separate rails, however this claim is often false; many omit the necessary current-limit circuitry,{5} both for cost reasons and because it is an irritation to customers.{1} (The lack is sometimes advertised as a feature under names like "rail fusion" or "current sharing".)
  
  
  ...


So it's apparently for exactly the same reason that electricians claim that different outlets in your house are on "different circuits", even when they are all electrically connected to each other.
Each circuit is allegedly current-limited with a fuse or circuit breaker.
And yet it is an irritation to customers when a circuit breaker blows because I have too much stuff plugged into it, when that same stuff works just fine if they are plugged into outlets in different rooms, or if someone replaces that fuse with a penny :-/.