You are correct, you cannot log the time.
The reason for logging your flight details maybe due to flight performance factors. i.e Pilot fatigue. Breaks for your next flight. Also it could be because of jump seat rides. After 9/11 the CAA and FAA do not like other people apart from to people flying the aircraft in the cockpit. I Know of rule and regulations in this area because I missed out on a jump seat ride with an airline in the past.
One other thing that I just thought of it could be flight pay. You no the score. Basic pay and the extra money when you fly the aircraft. This is why they may have logged your details to pay you flight pay for sitting there and watching
. you will need to read your new contact.