I always print my BP to have as a back-up option to mobile. Have done this ever since i was travelling the day WN's computer systems all went down network-wide 3 years ago. Anyone with a printed BP was allowed to check-in, clear security and board without much issue. Anyone without a printed BP had to wait in long lines for agents to manually check-in, hand write BPs/checked luggage tags, and go through an ID check at boarding. I know, this is only really useful for .0001% of the time but it sure is nice to have in that kind of unexpected situation.