The 30 pin dock connector had line-level audio output, as well as serial data lines for remote control. Back in the day I could plug my iPod or iPhone into my car and browse my music on a display on my dashboard and play back the audio over my car stereo. The dock connector also carried analog video (both composite and s-video), line audio input, firewire and was able to power accessories (3.3v) as well as charge the iPhone/iPod.

There was nothing at all at the time that could do all this using a single connector.