In days of yore, all bread was sourdough. So, it wasn't called sourdough unless it was real sour. The way to make it real sour was to let a sponge sit for many extra hours, preferably warm.
Many people do that today when making sourdough bread.
The process favors the acid forming bacilli, and lowers yeast activity. If you goof and the yeast activity gets too low, you can always throw in some bakers' yeast for the final rise. You also can get some rise by blowing the loaves up on a hot stone, in spite that a real sour sponge may not have much leavening activity. Denizens of yore had no access to bakers' yeast, nor did they have modern bread flours. The enzymes liberate sugars from starch allowing the rise to go on much longer than otherwise would be expected.