It's because nerds don't like to talk on the phone. They prefer email and other asynchronous messaging methods.

Here are cases I've used:

  1) Take a call while tethered (I think USians on AT&T underestimate this use case)

  2) Send an email while on a call a discuss to discuss attachements. (PDF, image mockups, etc)

  3) Send an iCal event and juggle schedules to meet

  4) Take a call while running with RunKeeper in the background

  5) Refer to Twitter DMs while on a call

  6) Sync DropBox/iDisk while on a call to talk about documents

  7) Look up directions on Google Maps for the caller

  8) Look up movies on Flixster and decide which movie to go to.

  9) Play games while on hold