Defoliate to:
Remove dying leaves
Remove leaves not getting any light
Increase airflow for mold protection
To give more light to buds
Outside, you dont have to worry about light problems much except at the very bottom of the plant, but if removing one fan leaf will open up an entire bud site, go ahead. Never remove more than 1/3 of your fan leaves at once and remember that the more you can keep, the better off she is.