Practical use of DC/DC-converters

Why and where is it quite useful to use the technology of dc/dc-converters and how can this simplify the whole power supply concept in a application? A modular DC/DC-Converter power solution or a complete discrete "in-house" designed solution, which is here the better way to realize the needs of the power supply design? Which major aspects are necessary to define the correct DC/DC-Converter, especially in complicated applications? Some quite interesting and special application examples, that show which major points where required to realize the final design wins at the customers application and which converter technology capabilities where necessary in the shown projects.