My thoughts are very similar to MarkG's...

Training your body to burn fat.
Weight control.
Getting yourself used to psychologically running on empty stomach.
Similarly a psychological boost when you do a similar distance race but fully fuelled, you feel you have energy to burn.

My overriding thoughts are that your fuelling and hydration benefits generally come from the days/weeks beforehand, not what you can squeeze in on the morning of a race/session.

But as AnthonyKay alluded to in his post somewhere above, everyone is different and has different needs... i've been training this way in various sports for almost a decade now and i wouldn't go back.

I went out on a 14 mile XC run on sunday morning... for breakfast had a swig of milk with some protein powder, and took a 500ml bottle of water, of which i drank approx half.