Wiggle chooses random values on certain frames (designated by the how often number), but in between those frames , it smoothly interpolates (just as if the random values were keyframes). Sometimes, you want After Effects to choose a new random value on each frame. For example, you might want to randomize opacity to create a flicker. Or, you might want to randomize the Direction property of the Directional Blur effect, so that on each frame, the blurriness runs in a different direction. Here's an expression you can add to any property which will randomize it frame-by-frame:
On each frame, this expression will choose a different number, between zero and whatever number is inside the parentheses (in this case 100).