What does "checked in" mean?
Many years ago checkin was something that happened at the airport not long before your flight was due to take off. The passenger assured the airline that they were physically present and ready to go. More recently, the person at the other side of the desk asked you as part of the process whether you were intending to carry anything nasty on board. if everything was in order, you received a boarding pass and were marked "present" in the register.
Now we are encouraged, very strongly encouraged in some cases, to check in and print our boarding pass at home, breaking the link between being checked in and being present at the airport.
If you can print one copy of a boarding pass at home, why not print several copies. In my family we had at one time three members with identical names, say "Fred Brown". What would happen if they made one booking, but printed three boarding passes, and then turned up separately at the airport with valid passports and other identification documents in the name under which the booking had been made. Assume a flight where seats are not assigned in advance and is likely to be no more than about two thirds full. Would all three be able to travel for one fare? My family couldn't do it now, sine two of the three "Fred Brown"s are long deceased, but I'm sure that a similar scenario cold be created without too much bother.
I suppose the machine-readable patch on the boarding pass could be, and very probably is, used to prevent such schemes, but what if a bit of judicious creasing and crumpling makes the code unreadable.