Why is this so important? Simple, on your wedding day, do you really want to put all of your faith and trust into the hands of someone who may not be qualified? Isn't that a big risk to take?
By being WPICC (the last 'C' stands for certified) your planner is able to ensure your big day runs smoothly, and you can rest assured that they are honouring the code of ethics.
If you'd like to read more about them, you can do so here.