The Cabin Crew Member was right..
Yes it is as she/he said, it helps balancing the aircraft. It is called Rule 1 or Rule 2, depends on how many pax there are and what result u want. blocking certain rows make u widen the CoG limits (front e back limits) just a little, to allow you to accept a ZFW calculated limit and not unload or move baggages or load ballast.Remember that even if a pax is considered 80 kgs without a luggage (93 before checking) depends how far he sits from the CoG to determine what influence he has on the overall aircraft balance.
i hope i have been helpfull,
best regards,
Damiano 738 FO