At the risk of sounding too obvious...they are for digital transfer of audio.
But what I'm sure you're asking is, "what devices would I use this with?"
Most low to mid level interfaces have a couple common digital connections. Usually SPDIF, ADAT, maybe AES/EBU.
SPDIF: (Sony/Phillips Digital Interconnect Format) This is many times a single RCA jack, but paired. One for in, one for out. It can also be optical, but many interfaces use the RCA connection for SPDIF. While it can carry a compressed multi channel signal, most times it's used for a stereo signal, so one stereo bus in or out.
ADAT: Optical cable. Nice tiny cable that can transfer multiple channels. Probably the most common use I personally see is to transfer ~8 channels of audio from an outboard preamp unit to an audio interface. The number of channels the cable can carry is defined by the device and how it handles sample rates. I have a unit that has two ADAT outs on it. I can use either one while running up to 48 kHz, but I need both if I run 96 kHz.
AES/EBU: Don't quote me on AES 100% and I apologize if I get some of it wrong. AES is a standard that encompasses a number of digital protocols. (Actually SPDIF falls under the AES3 standard.) As a very general rule of thumb, many mid level interfaces that use it...use AES/EBU which is used as a stereo pair as well. SPDIF is actually the consumer version of AES/EBU. It mostly uses XLR's, but there are variants on this too.
So, the cliff notes version:
AES and SPDIF: Stereo digital transmission
ADAT: Multi Channel
There is also MADI, which runs mostly through BNC connectors and can do WAY more than ADAT.
Brian J. Hallermann
Performing Arts Technical Director
Minnehaha Academy, Minneapolis, MN
Freelance musician/engineer www.superiorsound.biz