My "why" has always been: to moderate my own environmental impact by limiting consumption.
The great thing is this: if you set about to reduce your consumption of electricity, fossil fuels, and just material resources in general, you automatically spend WAY LESS MONEY. Because for the most part, consumption = spending = use of resources.
My household has a long way to go in improving this even further. And yet, over 15 years, the simplest, most intrinsically enjoyable and fulfilling ways of reducing consumption--biking to work, reducing utility bills, eating basic healthy local food wherever possible, being judicious about travel, avoiding mindless acquisition of stuff--have already led us to retire from full-time work 10 years earlier than we might otherwise have done. It just works all by itself. And we only really got into this around age 40.
So, limiting consumption but still living an extremely comfortable life, with some travel included, has brought our spending way below the local median income. And yet, subjectively, we feel we live like millionaires. And we no longer need jobs to sustain this for another 25-30 years. How would "more" of anything (more house, more car, more cool stuff, more whatever) bring us more enjoyment than e already have? We just can't see it.