How do we reduce corporate power if we don't stop buying corporate shit? We can't demand the government hold oil companies accountable and then complain about gas prices while driving home in SUVs.
The point is, people need to believe wasteful consumption is morally wrong. Because they think wasteful consumption is morally wrong they choose to reduce their consumption. And because they think wasteful consumption is morally wrong they choose to boycott and protest and demand legislation and put both financial and political pressure on corporations to change.
And because they live their principles every day they inspire others to share their views.
I grew up Catholic. I know how much a single vow of poverty can change the world. Show you care and you'll convince others it's worth caring about.