There's at least one really nice benefit: because there are no conductive wires in optical cables, electrical noise isn't directly coupled from the source to your DAC. This can make a big difference if your source is simultaneously charging from a noisy source like a car adapter [1]. Similarly, you won't create a ground loop by connecting your source to a wall-powered DAC.

[1] I have literally never seen an isolated [2] USB charger for a car, and I've looked. You can fudge it using an inverter, an isolation transformer, and a 120V charger. I'm not talking about "my golden ears hurt" noise; I'm talking about "egads this music sounds awful and crackly" noise.

[2] By isolated, I mean a charger that provides galvanic isolation between its inputs and outputs. Some switching power supplies are isolated and some are not. Any charger that plugs into a wall plug and isn't polarized MUST be isolated because, if it's not, it'll zap you. The upshot is that your car's electrical ground ends up connected to some piece of copper in or near your amp, and car's electrical systems are noisy as hell. [3]

[3] Fun reading: http://www.newark.com/pdfs/techarticles/tyco/Auto_Network_AN... If you ever visit the NRAO in West Virginia, there's a big area where you're not allowed to bring a non-diesel car because they're so noisy.