To most people, all four items in the What Computers See column are roughly equivalent. They're understood to be computer files of one sort or another. But when computers use these files on our behalf, they use them in very different ways. The first two uses enable people to read, print, and interact online. The latter two enable computers to exchange data without loss of fidelity, so that other people can read, print, and interact online.
The laws of information chemistry say that if we want to exchange data, we must provide it in a format that's useful for that purpose. In this example the PDF and HTML formats aren't; the iCalendar and xCalendar formats are. To most people it's not obvious why that's so. Our brains are such powerful pattern recognizers, and we know so much about the world in which the patterns occur, that we can look at Fig. 2a and see that the text clearly implies a structure involving dates, times, titles, and descriptions. Computers can't do that so easily or so well.
Fundamentally important to living in the future.