"Force you to think" is basically the idea, yes. The goal is to catch places where your high-level conceptual understanding of the task to be accomplished doesn

"Force you to think" is basically the idea, yes. The goal is to catch places where your high-level conceptual understanding of the task to be accomplished doesn't match the lower-level model of "how to get from here to there" you've mentally constructed, usually at weird corner cases. There's roughly three places where a programmer can make a mistake--understanding a task to be done, creating an algorithm to perform the task, and describing the algorithm to the computer. Type checks help insure that the algorithm you intended to code is what you actually coded; tests can help ensure that the algorithm you coded actually performs the task you think it does. "Beware of bugs in the above code; I have only proved it correct, not tried it." -- D. Knuth