A digital multitext can make it easier for readers to compare different versions of the Iliad; it can also enable new kinds of systematic, machine-assisted comparisons. For example: since we now have complete texts of the Venetus A and Venetus B manuscripts of the Iliad in the Homer Multitext project's Canonical Text Service, we can use the service's knowledge about the citation of each version to find the vertical variation between Venetus A and Venetus B (that is, what lines are present or absent in the two manuscripts).
The chart linked to this thumbnail image summarizes the variation book by book. The number of "plus" and "minus" verses are counted for each of the 24 books of the Iliad (the x axis); the dark blue section of a bar represents the number of lines that appear in A but not B; the light blue section represents the number of lines that appear in B but not A. (Phrased differently, if we are taking A as a reference text, and comparing B to it, we could say that the dark blue section represents "plus verses," and the light blue section represents "minus verses.")
Even a simple example like this creates a view of two manuscripts that would be prohibitively tedious to construct from print editions — and since there is no complete print edition of either Venetus A or Venetus B, would be impossible in any case. As we think about how to read and compare material in a digital multitext, we will have to go beyond our experience with print editions to rethink what it means to read and compare texts.
I'll reserve the subject of horizontal variation for another blog post.
Update: I've posted a slightly geekier but related discussion here.