Why

When you request for something, people are more likely to help if they think you have a reason for the ask. The validity of the reason, while important, turns out to be not as important as one would think.

The copy machine study corroborates this.

Which diet is good for the world?

A fun thought. These findings are from a single study, don't take them seriously. I read sometime back that in Australia, non-vegetarian diet is better since most of the meat consumed there is game meat. So, the best diet likely depends on location.

Source: https://www.elementascience.org/articles/116