It is important to already have an established practice so that you already know that the mindfulness journey is for you. If you were going to get a piano teacher, you'd like to be taught by somebody who already has some experience playing the piano! When training to teach you will be required to practice particular meditations and write diaries about them. A big part of being a mindfulness teacher is embodying what you teach, and your practice lets us know that you are ready for that.