Symmetry and clearance.
The gap between the tires and the fenders is larger in the front than in the back. It looks better if they are closer to the same measurement. Some people get a little overzealous and increase the height in the front too much causing the truck to look nose high. It may actually be level at that point, but it looks worse than before.
To allow for slightly larger tires to fit without causing the tires to rub on the fender well or suspension components. It is also cheaper than a full lift kit if you only want to increase the height by fewer than 4 inches.
There really isn't any other functional reason to level ones truck. Unless you have adjustable airbags in the rear, it's important to leave the front a little lower than the back to account for the rear end squatting when loaded.
This is why I have the snowplow prep kit on my truck. The springs that come with that seem to raise the truck in the front enough so I don't have to level it. It looks perfect.
I can't explain why anyone would want the front of the truck higher than the rear, which I think might have been what you were looking for an explanation on. It looks horrible.