What is the Vin Pin for?
The Vin pin serves the same purpose as the barrel jack.  The only difference is (usually) that it doesn't go through the schottky protection diode that causes a small voltage drop.

Normally its voltage is at around 0.3-0.4V below whatever you feed into the barrel jack.

If you power from USB then the USB voltage (~5V) is "back-fed" through the 5V regulator to appear on the Vin pin.  It's not safe to use that power for anything since you could damage the 5V regulator if you do.

The pin is designed to either be used to power external higher voltage devices with power (almost) direct from the barrel jack (motors, etc), or to feed power in from an external source without using the barrel jack (great for a shield that can provide its own power source).