Palette and painting knives are optional tools when painting however they are very handy to have. They can be used for mixing paint colors / painting and is a good technique for a beginner to try.
Knives come in a wide variety of shapes and sizes, certain shapes tend to lend themselves better to painting, or to mixing. These can also be used to scrape paint, either off of the canvas, or off of the paint palette. Again it is an artist’s personal choice.