In response to the original question on this thread, how about this for an answer:
A relative of mine was queuing to board a Johannesburg - London Heathrow flight on Saturday. When the boarding card of the Swedish woman in front of her was checked, it transpired that she was booked on the BA / Virgin flight to London, not the South African Airways flight that she was about to board. She was at the wrong gate. Much hysteria ensued and she was helped on her way to the right flight.
Situations like this one are the reason for additional checks. Hope that answers your question.