What are fan makeup brushes used for?

It's not really a necessary brush in your collection, however when you apply eyeshadow, you'll notice some colours are quite pigmented so they'll fall onto your face; in which you can then use the fan brush to 'sweep' the extra colour off your face.

You can use it to apply blush if you wish, but I'd recommend using a proper blush brush for that, as the fan brush might be too spread out, and less controlled.

Otherwise, use it how you like! Hope I've helped,

xoxo.