A cheesy title - 'Fan of The Fan Brush' but I am a very big fan of them. Out of all the brushes I have they are the ones that get used the most in every painting.
It's the smoothness of paint they help give that I like best. I've only ever used them with oil but I expect they're great with other mediums. They're brilliant for smoothing out brush marks. I'm no authority (and there isn't one) but the idea is to 'drag' the paint across the canvas with the fan brush. Dragging from a wet colour to another wet colour can give a really smooth faded effect. A lovely air brush blend can be achieved - sometimes. Even dragging wet to dry paint with a fan brush creates a sort of blend. Wet to wet paint though with a bit of liquin (making the paint very loose with oil paint) gives a lovely finish with the fan brush.
Of course fan brushes can be used sideways, other ways, what ever ways as well. Obviously creating other effects. They're no rules. Sometimes I paint directly with them. It's the blending and brush mark 'riddance' that I like best.
They are odd looking brushes. I think they stand out amongst others. This is seen in a shop where there's loads on display. The fan brush sticks out. They look delicate but aren't really. Not well made ones. They look like 'advanced' brushes for only the experienced to use. They're not really. I remember when I first bought them. I was very careful about drying them, then shaping them, careful how I used them. These days I'm a bit more blase and just use the things!
Fan brushes are great. I've only written this post just to say that really!
Cheers for now.