First of all, what is a DC-DC converter? DC-DC converters take DC power in and output the same DC power, but at a different voltage.
The main uses these days of these types of converters are in devices such as laptop computers and cellular phones. The general mode of operation of these devices is to draw their power from a battery. Batteries inherently can only provide so much voltage, and the electronics in these devices invariably require a different voltage than that of which the device’s battery is going to be capable. This is where the DC-DC converters step in to do its magic. These circuits do an amazing job of saving energy and optimizing battery use as well, often switching between different sub-circuits for various reasons. For example, as the battery drains, the voltage naturally goes down, and a circuit will kick in to rectify this. This eliminates the need for more batteries, or more cells in the same battery.
You will also see these type of circuits used a lot to increase the efficiency and viability of alternate power sources, such as photovoltaic (solar) and wind power sources. These are also sometimes known as power optimizers. These are absolutely essential to the feasibility of these devices.
You can also possibly find these converters in use in car radio, but these days it’s extremely rare. More often than not, you will find them mainly in classic cars, from the mid 70s and earlier. They were often used to get the appropriate voltage from the battery to places like the radio, for example. Modern radios are designed to need much lower voltage, however, so this is no longer a common application, but worth mention in passing. You will however still often find DC-DC converter circuits in use for other parts of the car, such as the brakes, (ABS or non-ABS), or the power steering mechanism.
You will usually still find at least one of these converters in every household appliance as well. Most of the time for small applications these types of converters are found in one single integrated circuit (IC), and are very cheap and easily replaceable. Care should be taken to use the exact same IC though, as these types of switchers are very complex electronically and not interchangeable at all with other models, in general.
This is a very important technology, and not one likely to go obsolete any time soon. As a matter of fact, we will probably rely on circuits like this more than ever as electronic devices increase in efficiency and complexity.