Why does a modern board carry many power rails rather than a single supply voltage?

Select one answer.

• Different parts need different voltages — logic, interfaces, a processor core, and memory each at their own level — and rails are also separated for noise, control, and standby, so one voltage cannot serve them all.
• Boards carry many rails purely for decoration and to look more impressive, since every component on any board could in fact run perfectly well from one single shared supply voltage.
• Extra rails are added only to waste the manufacturer's money, because a genuinely well-designed board would always use exactly one voltage everywhere on it and never once need more than one single regulator anywhere at all.
• The number of rails is chosen at random on each board with no reason at all, so knowing which rails exist and what each does can never help in diagnosing a power fault.