I suspect it forces the magnets to have a certain alignment. E.g. the bike light ends up pointing exactly in the right direction even when he just tosses it on

I suspect it forces the magnets to have a certain alignment. E.g. the bike light ends up pointing exactly in the right direction even when he just tosses it on. With one magnet and a steel plate you wouldn't have that behaviour. In some cases that might be a good thing, though.