Folks, don't none of you understand? You'll never be able to explain the function of yeast in a fermenting tank until you break free of the Great American Winner/Loser Concept. Good/Bad, Right/Wrong, Ebony/Ivory, Red State/Blue State, Conservative/Liberal, Nazi/Commie (except in the case of the Limbaugh/Palinites who can't seem to distinguish any difference).
When talking with someone who has a solid grasp on a black and white issue, how do you discuss shades of orange?
Whatever led you believe there's either NO yeast in the fermenter, or there's whatever yeast the distiller PUT into the fermenter? A vat full of water and fermentable grain sugar is very much like our own planet. It is teeming with yeast and bacteria, whole communities of them, each trying to grab hold of a piece of real estate, declare it "home", raise kids, and support their own soccer team. They consume the consumables, and they excrete their excretables. And each culture consumes different elements of the mash; and they excrete differently, too. Have you ever visited a foreign country? I mean, beyond the airport and the local Conte Naste resort complex? Cultures that eat foods that smell, well, diffently from what our food smells like also, uh... well there is certainly a different "air" about foreign cultures.
What the distiller (who is actually wearing his brewer's hat at this point) is doing with his "house yeast" is trying to dominate the "Fermenter Tank World" with a particular culture, so as to influence the total aroma/taste of the the effluent to fit a profile he wants to produce. There are several ways he can do this, some of which eliminate the differences and other that utilize the complexities they make available. Which method he chooses will have have a dramatic influence on the outcome.
(1) He can simply add a whole bunch of his own yeast all at once and hope that will crowd out all the others. It will almost certainly overwhelm bacteria (yeast is lots stronger than bacteria), but will probably be only partially successful in eliminating other yeasts. Still, if the dosage is sizeable enough, the end product will smell and taste more like the house yeast's doo-doo than like any of the others.
(2) He can ENSURE that his own yeast's flavor/aroma contribution is the only one by using sanitation and sterilization practices that eliminate all yeast cultures and prevent any but his own from taking hold anywhere in the tank. Since this also has the advantage of being the accepted production standard for food-grade products in most countries today, it is the one most commonly chosen for whiskey production as well. Never mind that it produces the same effect as replacing the "carmine", "scarlet", "cherise", "hot magenta, "radical red", and "razzamatazz" colors in a Crayola box with just "red". After all, if we still use the same labels, the kids won't know the difference.
Well, maybe at least THEIR kids won't.
(2) He can carefully add SOME of his own yeast, not all at once, but in stages, perhaps at particular temperatures in the process. Maybe some needs to be added to the northwest quadrant when the tank is half full, with more added to the southwest area when its nearly to the top. Done that way, the end result can be a concert of different yeast effluence, with the dominant note being the house yeast and much of the layered complexities resulting from the contribution of other strains.
I'll leave it to y'all to figure out (or eternally disagree about; or probably both) which of those any given distiller uses (or used to, anyway, back when he or his father had a choice)