There are still some airports in the world where route from the boarding gate to aircraft is not secure - therefore some carriers have a standard operating procedure to ensure the person entering the aircraft is entitled to do so. Of course this may seem frivolous when you are checked at both ends of a jetway that has no other access.
In the not too recent past I was travelling in transit through an international airport. I was one of the last passengers to board. The ground staff at the entrance to the jetway advised me to turn right and go down the stairs to apron level. At the bottom of the stairs there was no-one to direct me. I knew I was travelling on a 737 - so I walked over to the first one. Kinshasa? I asked the crewmember at 1L - 'no sir' was the reply. I found the correct aircraft at the third attempt. Luckily I have worked in and around airports for 35 years so I was very aware of the hazards of the ramp.
As long as this kind of situation happens at airports (where even the boundary fence is not secure
) there will be a need for the check at the aircraft door. The fact that some carriers do not do it - I would like to see their risk assessments.