The objects used to make the bower more intricate are not major sources of food or other useful material. They are merely decorative offerings, along the same principle that a Christmas tree with lots of presents is more appealing than one with none, even if the gifts are not for you. Ants, termites, and other social insects likewise demonstrate the spirit of giving to special members of the colony. The queen sits in her parlor day and night producing young while accepting a steady supply of gifts of food brought to her by the workers. And some male spiders give the ultimate gift, themselves, when they become the main meal for the female they have just mated with.
Some gift giving can go a step too far.
One common feature of gift giving among nonhuman animals is that the behavior is not altruistic. The gifts have a practical value for the giver. As humans, we have many animal traits that have been molded by culture over the ages. Sometimes the true origin and ancestral purpose of a trait become lost in the evolutionary past so that we think of ourselves as having risen above the rest of the animals. Maybe we have in some instances, and perhaps gift giving is one of those situations.