[...] towards the end of fermentation, yeast build up their glycogen and trehalose reserves; kind of like a bear storing fat for the winter. Glycogen and trehalose are two carbohydrates that act as food reserves for the yeast cell. Yeast slowly feed off these reserves when other food is not present, and use this food extensively to fuel the synthesis of essential lipids, sterols, and unsaturated fatty acids when pitched to an oxygenated wort. (Yeast will rapidly deplete their glycogen reserves when exposed to oxygen.) While glycogen can be likened to the fat that a bear stores for winter, the other component, trehalose, acts more like the bear's heavy fur coat. Trehalose seems to get built up on both the inside and outside of the cell membrane, and is generally believed to make the membrane structure more robust and more resistant to environmental stresses. By allowing the yeast starter fermentation to go to completion, these reserves are built up, and upon pitching, the yeast starts out with a ready fuel supply and a clean slate to better adapt it to the new wort. As noted earlier, though, these same reserves are used by the yeast while in hibernation, so if the yeast are left too long before pitching, the reserves may be depleted and should be replenished with a fresh starter wort fermentation before use.
I"ve had too manny homebrews for all that reading.. 
