For me, pole plants helped establish a "rythm" and helped keep my upper body moving independently from my lower body. You see a lot of beginners skiing in a rigid stance, where the whole body rotates back and forth. By incorporating pole plants into your routine, you can start twisting the upper body and getting a rythm going.
When I learned skiing, you wanted to keep the upper body aimed downhill, while the lower body twisted side to side in turns. The pole plant is sort of a natural "link" that ties it all together. By reaching for the next turn, you will keep your upper body forward and pointed down the hill.
And by the way, you don't have to "plant" the pole -- generally I just reach out and tap the snow with it. You will have some real laughable moments if you actually spike the pole into the snow....
I would start using pole plants as soon as you feel comfortable with your skiing and are ready to try something new. Good luck!