Elriko: According to some texts, after letting the wash sit for a while it develops pockets of higher and lower consumable content, while on the other hand developing other pockets which are over saturated with yeast. Simple problem of supply and demand!
Also, like Harrell mentioned, dissolved CO2 will raise the acidity of the wash and inhibit the metabolic processes of the yeast ,if not out right kill it with more sensitive strands,.
Turbo yeast for example is so aggressive and produces so much CO2 that this can be a bit more of a problem ,yet it"s also engineered to be more PH tolerant and includes balancing agents,.
Healthy live yeast also needs solid matter to latch onto and help it stay afloat in the wash, hence dead husks are not always a bad thing.
Stirring the wash up will help release the dissolved CO2, balance out the PH, balance out the "supply and demand" problem and also add matter for the yeast to stay in the central mass of things, over all, making for a cleaner, faster fermentation.
Yep, draw back is that you"ll have to wait some extra time for it all to settle the moment fermentation is complete. But that"s why those blessed scientists developed settling agents for us!