Early in my career when I thought of a palette knife, I thought of the guys on TV who used a knife to lay in half the painting in thirty seconds or to paint the same stylized tree every time.  Now I think of Richard Schmid (painting shown) using a knife to flatten a ground plane in the foreground or Trevor Chamberlain using a knife to soften edges or to flatten a dark plane against a light plane. The palette knife is a good tool to use when you want to simplify a passage in your painting or flatten it against a busier area.  When the paint gets too thick in the darks and the heavy brushstrokes cause glare, use the knife to flatten the thicker paint so it can't reflect the light.  A couple of strokes with a paletteknife can be a nice contrast against a bunch of short choppy brushstrokes.  I use the knife a lot to mix bigger piles of paint in the studio for large paints.  It's hard to mix big piles of paint with the brush. So go get a couple of knives ( small and medium size) but watch out for the sharp edges! 

You might also enjoy the YouTube video I did "When To Use A Palette Knife".  Here's the link: https://youtu.be/7yFHGto-gb8