If you opt out, you can't use automated immigration and boarding systems at many airports. At Singapore's Changi, for example, their process for entering and ex

If you opt out, you can't use automated immigration and boarding systems at many airports. At Singapore's Changi, for example, their process for entering and exiting the country is incredible, fully automated (unless the system determines you need further verification by a human, but this still saves more time than only human verification for everyone). This is the same for boarding, you scan your boarding pass and it verifies your picture with a camera, then you just get on the plane.