Diamond explains this in Chapter 7. There, he tells us that there is nothing particularly important about these two species. What is important is the general idea that some plants can be domesticated while others that could be used for food cannot be domesticated. This is important because it tells us why some plants in some places were domesticated while others were not. We need to know this because we need to understand, for example, that there was nothing wrong with the people who "failed" to domesticate acorns when compared with the people who succeeded in domesticating almonds. It was differences in the plants that caused the different outcomes. This fits with Diamond's whole argument that natural factors determine which societies will become more successful and more powerful.