There's a purely practical thing that these need to deal with: sometimes when you board you hand over the end stub and keep the large portion, and sometimes you

There's a purely practical thing that these need to deal with: sometimes when you board you hand over the end stub and keep the large portion, and sometimes you hand over the large portion and are given the stub .... This means that all (or enough) information needs to be on both bits .... You can't just put the seat number on one bit (or the flight number on one bit, as you may need it for customs forms later) etc etc