In digital reading as in paper based reading there is the need for the following (IMHO):
1. To be able to point at a position in the text independent of device and format. This is a need for scholars, but also for enthusiastic readers taking notes, students discussing a text book or for persons reading the same text on different devices. Hyperlink is great but it would be an advantage if the system could be and communicated of line and by humans as well.
2. To give the reader some idea of the length of the text (before reading) and the relative position (while reading). This is helpful information for readers planning their reading. Should I read In Search of Lost Time or Death in Venice?
The page numbers used in paper solves these tasks in a great way. For ereading it has been suggested to use percentages, paragraphs, kbs or some fixed page length. Sadly none of this has been agreed as a part of the epub-format. The current status is ebabel.