Well a high atom economy means that your process produces few waste products i.e. most of your products are useful or desired ones. Economically that is obviously good because you don't need to get rid of many waste products if any (atom economy can be 100%), and most of your products are useful so can be sold. Environmentally this is good because well you are simply producing less waste products.
I suggest finding a question on a past paper that asks about this in some shape or form and seeing what the mark scheme says also.