There are a few big reasons to prefer TOSLINK over other digital interfaces:
Cost: TOSLINK can be implemented with two plastic connectors (one an LED, one a photodiode). After that, you're in a 3.3V domain. Other interfaces require transformers and other analog electronics to go from the signalling domain to one that is useful in a digital system.
Noise/electrical performance: Using an optical interface means the systems are fully electrically isolated, with no risk of ground loops or noise coupling. There is also no risk of radiated emissions.
Cabling convenience: TOSLINK supports a very long, thin, cable that can be checked for functionality at a glance (look at the receiver end: is there light?). Any comparable electrical interface is going to be at least 3 conductors and require a multimeter to check for breaks.
Lastly, the reason to prefer digital is that every A/D-D/A step adds noise, distortion, and latency. The general desire is to convert to analog as late in the chain as possible to avoid that.
Cost: TOSLINK can be implemented with two plastic connectors (one an LED, one a photodiode). After that, you're in a 3.3V domain. Other interfaces require transformers and other analog electronics to go from the signalling domain to one that is useful in a digital system.
Noise/electrical performance: Using an optical interface means the systems are fully electrically isolated, with no risk of ground loops or noise coupling. There is also no risk of radiated emissions.
Cabling convenience: TOSLINK supports a very long, thin, cable that can be checked for functionality at a glance (look at the receiver end: is there light?). Any comparable electrical interface is going to be at least 3 conductors and require a multimeter to check for breaks.
Lastly, the reason to prefer digital is that every A/D-D/A step adds noise, distortion, and latency. The general desire is to convert to analog as late in the chain as possible to avoid that.