I recall a flight many years ago (I believe on a 737) that was only about half full. Seats were not assigned, but we as SLF found the first 15 rows or so taped off as not available. The cabin crew when asked indicated that the reason for having us all in the back half was balance management.
It was Boeing that came up with that idea!! If interested I could send you the details how we did our loadsheet with
adjusted weights.