It's more about expressing events in time than audio necessarily... Sometimes it's used just to keep other devices in sync with a tempo, sometimes it's used to control lights, and -sometimes- it tells an actual audio synthesizer when to start and stop making noise.
In 99% of occurrences, yes, Audio and MIDI can be intertwined, but in this particular project, I was using MIDI CC and PC messages to change preset and parameter settings on a rack mounted effects processor.
Oh, and we needed to use SYSEX a LOT in order to intercept clock timing messages, as well as complex data like preset names and multi parameter effect settings (EQ etc.). None of the messages sent/received affected music notes at all - it was all setting configuration only.
