Palette knives are useful in a myriad of ways.
-You can apply paint to your surface in a heavy impasto way. Or by using the edge you can apply paint in a fine line.
-You can mix paint on your palette.
-You can scoop paint or medium from a tin or pot.
-You can remove excess paint from your painting by scraping it off.
-You can draw in the wet paint with a palette knife.
-You can use it to scrape your palette clean.
-I find them very well suited for cleaning each other. I scrape the paint off of one with the other, under running water. This works even better with acrylics if they have sat in a water pot for a bit, the paint peels off easily.