Basis App note of modern DC/DC converters from Analog Devices. Link here (PDF)
Many systems require that the primary source of DC power be converted to other voltages. Battery driven circuitry is an obvious candidate. The 6V or 12V cell in a laptop computer must be converted to different potentials needed for memory, disc drives, display and operating logic. In theory, AC line powered systems should not need DC/DC converters because the implied power transformer can be equipped with multiple secondaries. In practice, economics, noise requirements, supply bus distribution problems and other constraints often make DC/DC conversion preferable. A common example is logic dominated, 5V powered systems utilizing ±15V driven analog components.
The range of applications for DC/DC converters is large, with many variations. Interest in converters is commensurately quite high. Increased use of single supply powered systems, stiffening performance requirements and battery operation have increased converter usage.