There are 3 reasons I sometimes paint with the palette knife instead of the brush...

1.  It goes quickly.  If I'm out on location and the weather is changing, or I'm running out of time, then the knife gets things done quickly.  (the drawback is that while the knife uses time efficiently, it does not use paint efficiently, so I always use loads more paint)

2.  It can create a lot of visual interest in an otherwise ordinary scene.

3.  It helps me stay loose.  Sometimes when I feel that I'm fussing over my paintings too much, putting aside the brush in favor of the knife can really help.  I give up some control but stay focused on the main intent of the painting.

Actually there are 4 reasons...#4.  It's fun!

This painting was one that I planned on painting with the knife right from the start.  I did use a brush to lay out the design and block in the main shapes.  After that it was all palette knife.