Well, nothing so involved as a tutorial, but it's handy when you are using MIDI between programs on the same machine. If OMS only saw hardware MIDI ports, and you wanted to pipe events from a standalone sequencer app into a standalone synth (for instance), you'd need to physically route a MIDI cable from an out to an in. Which is obviously inefficient, since within the box there's no need to send the data out through a UART and parse it again. The IAC bus handles this within the same computer, while keeping your physical ports free.
There are IMO some great reasons to do this. DAWs tend to be very limited in their approach to handling events in time - essentially being a simulation of a multitrack tape studio. This is great for step sequencing, but horrible for interactive sequencing. Many of the more interesting sequencer programs focus only on being a sequencer. On the other end, it helps to get event data into non-plugin synths, either programs or PCI cards.