Convert a non-isolated converter to an isolated converter

To expand on what Martin said, you can also buy pre-certified "power bricks" that are like the wall wart except are intended to be internal to your unit. Most of these nowadays handle universal line power in. You provide a universal line cord socket on your unit. That connects only to the power brick with a fuse in series. The other side of the power brick provides isolated and regulated DC.

The power brick market is very competitive. These bricks don't cost much more than a big iron transformer to handle the same power. Mean Well, Astec, Artesyn, and CUI are just a few companies in this space that come to mind. Do a little digging and you can find more.

If you have high currents or frequencies riding around one part of a circuit that you'd like to protect another part of the circuit from, isolation is certainly an option. For example, if you have a microcontroller driving a high-current motor with PWM, you can optoisolate the PWM pin on the microcontroller, then power up the motor circuit with an isolated DC to DC. Doesn't work if you don't optoisolate (or isolate in some other way), obviously.

If you're building a medical device with patient leads, isolation of the patient leads might be critical to meeting safety standards. An isolated DC to DC, along with isolation of all the analog and digital signals that need to connect to the patient would be in order.

Note that an isolated DC to DC becomes a nonisolated DC to DC if you connect the grounds!! Also, if you're putting test points into your board, be sure to include one for every ground, and don't forget that its not hard to connect isolated grounds with your oscilloscope ground clips accidentally!