The palette knife is not just a tool for mixing paint on your palette. You can actually use the palette knife to create some pretty stunning effects in your paintings which are not possible with a brush. Some artists ignore the brush all together in favor of the palette knife, it is so easy to clean a palette knife. You literally just need one wipe with a cloth or paper towel and all the paint is gone. This is a huge advantage over brushes, which require much more care.
It can be cheap, depending on the quality of the palette knife.
It is great for building up a thick texture and creating a broken color effect.
You can use the edge of the palette knife to create sharp lines.
The bold strokes created by the palette knife can complement the more delicate strokes. You are not able to paint with as much accuracy using a palette knife.
It is not as versatile as a brush.