Out of all the makeup brushes, I think the fan brush is the most underused and overlooked brush among the bunch. We tend to neglect the fan brush because we just see it as a sweeper brush. It's something you use to remove excess powder or fallen pigments of eyeshadow. That part is true although it makes you realize how useful it is. But see, it's also very versatile.

I admit I'm one of those people who reach out for the big, fluffy face brushes mainly because they do the job of powdering quickly and effectively. But just because a brush isn't big and fluffy doesn't mean it can't do the same thing. If you look at the fan brush, it's thinner, flatter, and more splayed out than most face brushes. It's also a tad stiffer. But that's why it's so helpful in powdering. Because it's thin, you get to reach those hard to powder areas like under the eyes and the sides of the nose. It isn't impossible to powder those areas with bigger brushes but it's quite a challenge. Also, the fan brush picks up an adequate amount of product which I really love because it helps avoid an overpowdered/cakey face. The fan brush can also be used for highlighting. You can easily highlight the nose bridge, top of the cheeks, and the brow bone, thanks to the size of the brush. Some fan brushes are thicker and softer, so those kinds can also be used for applying blush. Fan brushes also come in smaller ones and are mainly used to apply mascara. 

See, I told you fan brushes can be versatile! I may not use it most of the time but when I do, it always reminds me how amazing it is. Some may love it, some may say meh to it. Either way, you really can't say it's a useless brush, can you? Because if you do, you're totally lying. :P