When to use it: To apply powder highlighter or to clean up any stray flecks of makeup.
How to use it: Lightly sweep the brush across the tops of your cheekbones and down the bridge of your nose, as well as, wherever you need to brush off any fallout from your other products.
Why you use it: The fanned-out bristles spread your highlighter evenly onto your skin so you get the most natural finish. And if you’re using it to clean up excess powders, it acts like a tiny little broom.
This handy brush can play the role of powder, bronzer, blush, contour, and highlight brushes at the same time. So, if you want to buy less brushes, this is the place to spend a bit more money on ONE REALLY GOOD brush that lays on color beautifully and does it all. I have listed some of my favorites below.
Merge with:
This is one of the most versatile brushes to use because the flat top and generous amount of bristles make it easy to blend out any type of product (save for eyeshadow). It is used to apply powder, liquid or cream foundation, blush, highlighter or bronzer—or as a finishing step to blend everything together. 
How to use it: In circular buffing motions, starting at the center of your face and moving out toward the perimeter.
Use an eye shadow brush to easily apply primer and eye shadow. The compact size of this brush makes it easy to blend colors while still keeping the precision of eye shadow application.
These are not a brush, but I included them here because I love applying my foundation with these little sponges. They are the perfect fit for my face and get all into the nooks and crannies for a flawless application. They are a great price, making them easy to toss after just a few uses. You always want to use these sponges damp, or they will soak up your foundation and not work well. I also like to spray them with setting spray to make my foundation last longer.