A DC-to-DC converter is used to produce a galvanically isolated voltage supply of a value you don't already have in your system. For example, if you have a 5VDC system and need to generate +/- 10VDC signals, you can use a DC-to-DC converter to produce the +10VDC and -10VDC. (I know - You can actually use a charge-pumped MAX232 chip to do this, too - but it's just an example).