Airflow and light penetration are reasons to defoliate the plant.
I do a preflower defoliation, a 21 day post flower and if needed a 42 day post flower. Removing leaves that block airflow and light penetration.
In all my years of growing I have gone too far a few times and probably lost some yield but there is the balance.
This is a Monster Crop I did in 2018. This was a clone I took at day 32 of flowe
The only one out of 11 clones to root. Pic taken on 1/21/18

Two weeks later. 2/05/18
I had the mothers of this plant in the tent in flower so this is was the day they went into the tent.
4/03/18
Tied down the plant 4/21/18

Over the top defoliation. 4/22/18

Well I said it was over the top but here she is 7 days later.

And here she is the day before harvest.

Got 19 plus zips of smokable buds off the haul.

This over the top defoliation probably caused the overall weight to be a bit less, but the buds were totally removed from the branches and totally trimmed for smoking, so I was not disappointed with this grow.