most times the results will be contaminated by bacteria. The knowledge of what was actually going on when you do this was only gained in the past 250 years or so (before that, yeast was known as godisgreat).

In addition to doing it the way you described (put out flour and water in a bowl and wait for it to bubble and smell nice), you can also buy a $100 microscope, collect wild yeast, clean the bactiera off, and grow mass cultures. Sometimes it pays to work at the last step of the industrial, scientific and agricultural revolutions.