What are the S/PDIF, AES/EBU and Optical connectors used for on my device?
The signal on the 1/4" and XLR connectors is analog in most cases (more on this later), at the S/PDIF, AES/EBU and Optical ports it is digital.
When connecting to a digital processor, the analog signal is converted to digital and processed internally. This Analog/Digital conversion is performed by the Analog/Digital converter, also known as an A/D converter. Then, after the signal is run through the effect in the device, the signal must be converted into analog again to be heard. This conversion is performed by the Digital/Analog converter, also known as a D/A converter.
The reason you may consider using these digital ports are because in every AD/DA conversion there can be degradation, this can be heard in the dynamic range and frequency response. The best way to avoid these conversion issues is to convert the analog signal to digital as soon as possible and keep this digital signal throughout the signal chain, converting the digital signal into analog only at the end of the chain. This will give you the best quality of sound and avoid issues that may be produced by multiple conversions.
Where these digital connectors are most useful is during a recording session. In many typical modern recording sessions the input source, being either a microphone or instrument signal is plugged into a computer through an audio interface. The input on the interface is the A/D converter. The analog signal is converted to digital so it can be recorded and processed in various ways. To hear the result, the signal is once again converted by the D/A converter on the audio interface.
These digital connectors can also be useful when chaining multiple effects processors together in other scenarios like live performance where multiple effects might be part of a performers rack of effects. In this case the same principles apply as previously discussed though the application may be different.
We can find other digital audio formats: TosLink/Optical, AES/EBU including XLR and D-Sub and Optical S/Pdif over 3,5mm jacks( most typical on computers). Many AES signals us an XLR connector though the signal is digital. In this case be mindful of the labeling on your unit. The RCA connectors that are used on these devices are also used for analog connectivity on some consumer devices, these connectors is rarely seen on a professional unit for analog connectivity.
If you are interested in reading further about this topic, please reference this Wikipedia link: http://en.wikipedia.org/wiki/Digital_audio