A DC-DC converter is a power electronics device that accepts a DC input voltage and provides a DC output voltage. The output voltage of DC to DC converter can be greater than the input voltage or vice versa.
The connection and disconnection of power supply to the load can be controlled using a switch. DC to DC converters are used to provide DC regulated and constant power supply to electronics devices.
A DC-DC converter allows you to use your 12V pump on a 48V system, for example!