Extra nesting material
It’s a little late for this Gannet to start bringing back more seaplants to the nest. Their chick is already massive and there’s no egg to worry about having an unfortunate roll off a cliff.
Maybe it’s to mark-out clearly their nesting territory against encroachment by a fellow Gannet. Maybe it’s an important part of pair-bonding and a token of love between the parents. Maybe it’s just because they enjoy it.
Whatever their motivation, they make a spectacular sight.