As a new instructor, only having a year of teaching experience, I have learned that planning and preparation is not only for the students, but it's for the instructor as well. It helps you to be more organized with your delivery, makes you seem more professional and competent, and it keeps the students busy and occupied for the full length of the class time. It is always best to overplan than underplan because students may finish an assignment or activity much sooner than expected. If this happens, instead of having to think on your toes about what to do/say next, you'll already have another assignment or activity ready for them, or you'll already be prepared to go to the next topic on your course outline.