If you forget to check in online or forget or lose your boarding pass, you will still be allowed to travel. You will just have to check in as normal or have a new boarding card printed for you at the airport.
Do I need a physical boarding pass for international?
There's also the consideration of border control if you're traveling internationally — sometimes border agents might ask to see your boarding pass, and it's usually easier to hand them the paper version rather than your whole phone.