It's allways nice to know where you'd be going. A useful gadget to inform you about that while riding your bike, can be a bike-bell with build-in compass. In that case you can see in what direction you're going without having to stop and dig up your compass from somewhere where you thought you had it stowed away. Usually the last place you are looking in.
Is it usefull, such a bell? Well, when you are about to start your weekly workout tour and want to now from which direction the wind is blowing, it is. Otherwise, when your cycling a multiple days trekking tour, I doubt that, because in that case you've allready planned basically the route. But when cycling a daily tour starting from your home, it can be usefull to adjust your route to have tailwind on your the last leg back home.