The product of (ATP, a 'sugar') of photosynthesis is mobile. Do not remove leaves because the herd does it blindly.
improve air flow if concerned about microbial growth, sure, but not whimsically or because someone beleives reducing photosynthesis potential from the plant is somehow a positive thing. Energy is a currency. Reducing that potential is a net negative in most contexts. Avoiding disease is a good reason to consider it, but there are others, i'm sure. It should be overtly obvious when it is needed.
The way it is practiced by the lowest common denominator, defoliation is too often akin to masturbation. nothing good for the plant (often a negative impact) and 100% just to make the individual feel good about doing "something", LOL