The huge ocean spirits travel with the seasons, rarely coming near the shore. Only magical folk can see them, and even then only in certain conditions. For a long time they were considered ill omens, said to bring curses upon sailors who saw them. We know better, nowadays.
What they do is a mystery, but not all questions need an answer. Not everything in this world works around mankind, magical or otherwise. Some things are none of our business.
It’s human nature to want to understand things, but with these giantesses of the sea, perhaps we should learn to appreciate their mystery, their disconnection from the world as we know it, and stop overthinking things.