Some speaker setups (such as the Altec Lansing ADA885's, I say these because they're the only ones I know off the top of my head that have it) have the single digital mini-jack as a plug in rather than using the 2 sets of plugs for front and rear speakers, which is an analog input. Using the digital jack you can output a digital signal and support digital sound from your DVD's if you wish. There aren't a whole lot of advantages other than not having to use 2 sets of analog outputs in 4-speaker setups.
divinemartyr