My understanding is that the purpose of scanning the passport MRZ or NFC chip (instead of just typing in the details manually) is a way of verifying that you actually HAVE the passport in your hand within 24 hours of travel. People who are not frequent international travelers might not remember about the need to bring the passport to the airport until it's too late, and then they miss their flight. So, the scan forces you to actually be holding the passport and thinking about it. In addition, online check-in (on the website) requires you to re-verify passport details by typing them in (which, for most people, requires looking at the actual passport), but in that case, you have to see an agent to verify documents, as noted above.