IAC DRIVER

I tried to find the right forum but I couldnt find it. My question is about IAC driver. I know what it stans for and also the main purpose "to send midi from application to application in the same computer.
Can someone explain to me with e real example when you can use it?
I am a composer and use digital performer as my sequencer and I always had this question....what is this IAC driver and how to use it?
Thanks