That type of bread stems from the days before you could buy dry yeast in packages. If you didn't have your own yeast culture growing at all times, you didn't have bread. Women kept a "starter" of yeast growing all week, and usually once or twice a week they baked bread, being sure to set another starter back. My grandma called it "everlasting yeast" and she did it from the day she was married in 1924 until the day she died, a few years ago. My mom has the little crock that she kept her yeast starter in.