I don't think I've ever used DigiD to verify my passport, I was vaguely aware it had the capability though. On the other hand I use DigiD all the time to login to websites. My health insurance, government websites, etc. Super efficient and simple.
The passport feature is a new one to provide an alternative safer method of verifying ID for the times you need it. It isn't the default use of DigiD and is meant as an alternative to physically taking your passport places.
The passport NFC scan is only intended to authenticate with DigiD once. Basically to connect your DigiD account to "you". In the past this was done by them mailing you a letter with a password in it to your registered place of living.
The passport feature is a new one to provide an alternative safer method of verifying ID for the times you need it. It isn't the default use of DigiD and is meant as an alternative to physically taking your passport places.