For many, many reasons. But mostly for the gas savings.
When I lived with stbx, I would fill up on Sundays and again on Weds. nights or Thurs. a.m.s to do all of my weekly work/daycare/life commuting. And that was the basics - no extra trips for fun activities on the weekends. I was spending $5-600 in gas in a month, easy.
I filled up on Sunday afternoon after I ran my last errand. It is Thursday and I am just now hitting half a tank. A whole week on one tank is a MIRACLE!