In the case of the first point:   the idea here is that, in order to raise an exception, the base-class will call one of its methods.   The descendant classes will then override those methods.   Hence, each descendant class can arrange to throw a particular Exception ... and/or to otherwise distinguish their response from everybody else’s.

In the case of the second point:   well, it is extremely easy indeed, when faced with the obstacle of a thick iron door, to focus your entire attention upon how to drill through that door ... and, thus, to completely overlook the fact that the door is set into a wooden wall, next to an open window.   (Believe me, it happens to all of us.   That's why Homer Simpson is always whacking himself on the forehead and saying:   “Doh!!”)