PaperTiger
Right. So the 'security' aspect of checking the BP again at L1 is to identify those passengers who have materialised or dematerialised in the jetway ?
As you probably know, Jetways often have external access at the a/c end. There is a side door that goes to external steps and so to the apron enabling staff to move quickly from supervising the loading to speaking to a/c crew. Many perimeter fences are not secure (in many countries) so you must check the BP.
However many announcements (written and spoken) there will always be a small percentage of folks who do not have the BP ready. On a recent trip, a family in front of me going through security ... standing in line, I could easily see 'mum' collect the children's passports for safe keeping. Once through into the duty free, 'dad' asked 'mum' for the passports for safe keeping and then panicked that one of them was missing. I had seen that one of the children had retained theirs but it took the family nearly a minute to work out whose was missing and where it was. So, not having the BP to hand is a trivial exercise.