It would be for a very specific situation to connect a coax digital output to the digital optical input of my Sony D100 PCM recorder. This is the only digital input available. The only other option would be to convert the coax digital to analogue and then back again.
I think mine came out at around £11 (from the big river). If you want a much higher quality converter then something like the Mutec MC-1.1+ may be worth investigating.