I use SPIDF to slave the clock of one device to another where that is the only method to slave the clock other than midi sync.

Bonzos Ghost
I use S/PDIF for connecting a digital mixer to my daw interface,

I'm doing this also, s/pdif out from my interface, into a digital mixer, s/pdif out into the monitor amp

Thank you all for the answers! I don't need it yet, but when I do I'll know it now.

I also use a coax S/PDIF connection in order to give me two more analogue inputs by connecting my Roland MMP-2 mic pre to my OCTA-CAPTURE. I also have a Lexicon MPX 500 and an Oberheim OB-12 which also have S/PDIF outputs, but I've not tried those yet...
 
So, they are there for using, if you need them

Coax SPDIF is weird.
 
I currently use it to route audio from the interface to my Roland V-Synth, as an input for sampling.  It works well for this task, and I don't have to sync the interface to the SPDIF.  I have used it before, as an output from the Waldorf Q, as an attempt to free up some analog audio channels on the interface, but the audio was glitchy and terrible, because the Q must be the master clock.
 
And FYI: SPDIF coax cables are relatively cheap.  Many on Amazon, just get one that is 75-ohm impedance.

I use SPDIF to connect my Fractal Axe FX II XL to my UAD  Apollo. Nice to record without any AD/DA conversion in the middle.

2:43AM
Coax SPDIF is weird.
 
I currently use it to route audio from the interface to my Roland V-Synth, as an input for sampling.  It works well for this task, and I don't have to sync the interface to the SPDIF.  I have used it before, as an output from the Waldorf Q, as an attempt to free up some analog audio channels on the interface, but the audio was glitchy and terrible, because the Q must be the master clock.
 
And FYI: SPDIF coax cables are relatively cheap.  Many on Amazon, just get one that is 75-ohm impedance.

FYI: SPDIF carries clock info