Two main cases:
- When calling code has asked you to do something unreasonable.
- When outside dependencies have left you with nothing reasonable to do.
In the first case consider a method int IntegerDivide(int dividend, int divisor). Now, we all know we should catch conditions like dividing by zero ourselves. However, if the calling code has asked this method to divide by zero, it's too late - it's not our code that is deciding to divide by zero, it's the calling code. The only reasonable thing to do here is to throw an exception.
In the second case, consider a component that reads a file and returns some sort of computed result, and what it should do when it fails to open the file. If we had access to the user interface, then we should catch this failure and present a useful and informative message to the user. However we don't, as we're writing a component for use by other code closer to the UI. Again, the only thing we can do is throw an exception (and hope that the calling code does the right thing when it comes to informing the user).
