Games need to find a balance between predictable and unpredictable gameplay. If a game is completely predictable then it becomes uninteresting because there is no challenge. If a game is completely unpredictable then it is nearly unplayable because any action a player makes results in an unintended action. When there is a balance between predictable and unpredictable, the player is able to make decisions with an expected result and strategize for the elements of the game that aren't predictable. This makes for gameplay that makes a player want to keep playing.
Example #1: Angry Birds
Angry Birds is great because of the way it handles its physics. The birds and pigs act in relatively predictable ways. At the same time, though, its quite unpredictable because two shots rarely result in the same thing. There is always a bit of unknown when it comes to how the birds will interact with the environment and collisions.