When one is in the art store staring at rows and rows of brushes, it can be overwhelming. I know a lot about brushes, but I too will often just buy new ones to try them out and expand my tools.
Fan brushes always attract the eye when brush shopping because they are so different than most other brushes (I did see a 'brush' made of a bird's wing once in an art store in Santa Fe).
They come in various sizes, but they also are made of different bristle materials and that is what I want to tell you about.
First you can get a fan brush in hog's hair bristles. The bristles are stiff and are good for dragging wet paint that you have applied to get certain effects. I have used this brush to pull paint over the "edge" of a painted cloth and it helps to show the change of direction of the cloth falling over the painted edge. Sometimes I just pull the applied paint while it is still wet and sometimes I use a slightly different color and put it on top of the color already applied.
Secondly you can get fan brushes made of very soft acrylic bristles. These are not good for the job I mentioned in the above paragraph. They are for blending paint in the most delicate way.
I would suggest you try out these things on a sample board or canvas paper til you get the hang of it.
Also, if you pick paint up off your palette on your fan brush, don't pick up big clumps of paint. You made need to thin the paint down a bit with linseed oil (or whatever medium you are using) so that the paint will flow off the brush easily. You will have to reload the brush often, sometimes with every stroke. Notice that a fan brush is thin and is not made to hold a lot of paint, therefore you have to reload often.
You can use part of the fan brush, the side edges. You don't have to use the whole thing. Sometimes you need to use one of these but the entire fan shape is too much for the job. Just load one side of it and do the job!
Tip: if someone can tell you used a fan brush for effects in your painting that isn't a good thing!
You don't want it to look gimmicky. Keep it subtle.