There are many possible reasons. You might enjoy the feeling of independence or self reliance that supplying your own power brings. You might be a weed grower and not want to be found out because of your energy bills. You may feel that it's more responsible to produce your own electricity and have a clear understanding of its environmental costs. Grid power may be unreliable in your area, or expensive. Maybe you think that power lines give you cancer. Maybe grid power is simply unavailable in your location.