There is an interesting story behind this brush, usually called a fan brush for obvious reasons. Back when I was in Art School we learned that the fan brush was bad, basically a crutch for poor artists. Students who were unhappy with their work would go back over it with a fan brush, like how you would apply a filter in Photoshop today. This would smooth the painting out, and give it an “impressionistic” feel instead of a “badly painted” feel (or at least, so they hoped). As such our teachers found it highly irritating, and told us to never use them. With the experience I have now, I can see this attitude is a little questionable, and I understand what the fan brush is actually good for.
Brush strokes are very visible in oil paintings, and the fan brush can be used to smooth paint out into a flat color. Normally you would want to use your brush strokes to add more depth to your painting, but sometimes having a flat colored surface is very useful. Particularly with the new technique I’m using, where I’ll be painting onto a dry background. If the background has texture I won’t be able to blend that texture into the objects on the background. It’ll work against me, so it’s best to have a flat background, created by the fan brush. As a tool it is best used for removing the evidence of your brush strokes, not obfuscating your amateurish mistakes! The fan brush probably has even more profitable uses that I haven’t discovered, early prejudices can be hard to shake.