Batteries, OK, agreed. But...
For "power supplies", you might as well use a USB charger cable correct? Are there places where someone has an advantage using a different-voltage cable? (because after all, as you mentioned, it's being regulated to 5V).
Yes, but by being able to provide a higher voltage via the external power connector (7vdc to 12vdc) then that higher voltage is available via the Vin pin to power higher voltage components that the arduino can control, say small 12vdc motors, etc. So in some cases using a single higher voltage source lets the arduino on-board regulator supply the +5vdc, as well as having higher voltages to power other stuff controlled by the arduino and therefore is more convenient.