A power supply’s source, in actual use, is never ideal. The real behavior, including parasitics, must be considered to build a reliable power system. When we use power supplies, we ensure that a dc-dc converter, such as a switching regulator, can withstand a certain input voltage range and generate the required output voltage from it with sufficient current.
The input voltage is frequently specified as a range because it’s usually not regulated exactly. For a power supply to function reliably, though, there must always be an input voltage within the permissible range available to the switching regulator.