When you sit on your bike, you put more weight on the rear wheel than the front (it's easy to use bathroom scales to see exactly how much). By distributing your cargo 60/40 to the front, you get a more even distribution of overall weight on the tires. This is good for several reasons. The tires (assuming same size) have the same load bearing ability, so having them evenly balanced is better - particularly on a heavily laden tourer. Also if you put to much weight to far back, there is no weight to hold the front down. This negatively affects cornering, inducing uncontrollable under steer, and also affects hill climbing. A tourer usually has an insanely low granny gear, which means it's possible to grind up very steep hills, provided the torque does not lift the front wheel. Pulling wheelies on a fully loaded tourer is something best avoided by all but the most skilled riders.
These are only guidelines. Every rider and bike is different, so you should do what suits you. Don't be scared to experiment - even once you find a way to load that you like. If you don't try something different, you may never find that "better" way...
Forgive me, I will go for the bonus points.
Max Torque induced by Pedal
Force on Chain =
(Rider Weight * Crank Length) / Small Chainring Diameter.
Torque on back wheel =
(Force on Chain * Diameter of Largest Cassette gear ) / Wheel Diameter
Torque to rotate bike =
Wheel base * Weight on Front wheel.
Crash === Torque to rotate bike > Torque on back wheel