Question

what is the benefit to society if everyone reduces the amount of meat they eat?

Answer 1

Well there are both benefits and negatives.
The benefits of it is that people will stop selling so much meat which also means that they stop killing so many innocent animals. 
People won't be so overweight (aka fat)
there are some benefits.

Answer 2

there'll be a decrease in health risks and problems for one... Hope it can help