Easy way to think about castor is a bicycle since everyone has played with one. The reason you can ride a bike with no hands is the front fork being tipped forward toward the wheel. That angle gives a stabilizing effect on the steering system. Your front axle has that same angle built into the balljoints for the same reason.
Imagine how squirrly the bike would be if the head tube was straight up and down. You'd have to hold the wheel straight all the time and anything that came by would try to turn it. Same thing with a truck if you tip the pinion up too much.
In a truck you get "good" castor anytime you tip the bottom of the axle forward, like adding length to the shackle. We really add the length to keep the spring from hitting the frame but the extra castor is typically a good byproduct too. It's a 1/2" of length so you'll get 1/4" of lift from it.
Typically you want 4-6 degrees of castor and on a leaf sprung truck a little more won't hurt since the springs will try to wrap the wrong way on braking and the pitch angle of the vehicle takes castor away on hard braking also. A little more castor can make it so the steering doesn't get weird on hard braking.
There's my book on the subject. I get carried away quick.
