From
the Fresh Loaf:
While you were away the flour was absorbing the water and the gluten strands have begun to develop. Now you can mix in your preferment, your salt, and the remainder of your yeast and, with very little mixing, achieve a high level of development with considerably less work. The crumb of your dough is also likely to come out much whiter since it has not been highly oxidized by all the beating and whipping.
Better bread, less work. What's to complain about?
Read about it here:
http://www.thefreshloaf.com/lessons/tentips_8_autolyse
