While skating you usually stand on top of the board to start a trick. In freestyle this is also the case, but all sides of the board are used for tricks as well. For example, while in railstand – a common access point for further tricks – the board stands on the ground flipped 90 degrees. The side of the deck (rails) touch the ground and the total width of the axle setup provides a stable stand.
What does this have to do with the wheels? Very simple: With usual skateboard wheels, part of the axle sticks out bit to the side of the wheel. With in a railstand, you not only stand rather unstable, you also would ruin the thread on your axle.
Freestyle wheels get rid of the problem of the axle by an offset. The point of contact inside the wheel for the bearings is not in the middle as in other skateboard wheels, but slightly offset towards the board. This creates more space for the axle pin and the wheels cover them all together with the nut, thus providing a stable and secure stand on the side of your board.
In a nutshell: Wheel hides axle, axle doesn’t rest on the ground in railstands, ground only touches wheels. The causal chain of stability – learned something new today!