Environment: by shopping local you cut costs and waste. Products requires less transportation, which requires fuel and creates pollution. Also, because locally owned businesses have a personal stake in the area, they tend to be much more conscientious about how they treat their local environment.
Economy: More money stays local. More jobs are created locally. More tax dollars stay local. More money is invested in the local community. More money stays in the hands of average people (no million dollar bonuses here!).
Community: Locally owned businesses tend to have a much more personal relationship with customers. This creates strong relationships throughout the community.