For light penetration and airflow.
Leaves are like solar panels and if they are covered, they are not working very well.
I normally try to clean the bottom and top and leave a layer of leaves in the middle.
My 2c with defoliation is cut the leaf a few mm away from the stem and the leftover bit will dry out and fall off, if you cut on the stem or pluck, you can leave a bigger wound that can lead to trouble.