But there's really good reason for this. On the app it can use NFC to read your passport data exactly. Until WebNFC supports reading passports, it is a much more efficient way.
It's not like they are getting some long term benefit of having the app on your phone. It's just because WebNFC can't read passports.
The information on my passport is of comparatively little value compared to the information on my devices. Most states could get my passport information with little more than a friendly request to my government, same for most, access to my phone however.
Why give up more information than is strictly necessary, so you can tap your passport on your phone? Not convincing imo.
Because for many people with poor eyesight, poor English or computer literacy tapping a passport is far easier than typing the data in with no risk of transcription errors.
It's not like they are getting some long term benefit of having the app on your phone. It's just because WebNFC can't read passports.