Have a look at your ATX PC power supply. See the voltage switch?
View attachment 309149
There's two ways of doing 'world' voltage on SMPSs. Automatic and manual. Manual is safer, more reliable and cheaper, but it requires a modicum of intelligence from the end user. Sadly, that is missing in a significant proportion of adults the world over.
Automatic requires relays/switching and trigger levels/hysteresis along with added complexity.
Basically, all modern SMPSs are designed to run primarily on 220-240V with switchable voltage doublers for 110-120V. That's why, they will offer better performance in a lot of cases on world voltages (220-240V). Losses (I²R) are greater at lower voltages like the US's 120V.