When making a starter, the intent is to make yeast. We use stirplates in part to continually oxidize the wort, enabling the yeast to multiply.
In this question about starter oxidation, one of the answers talks about lag transitioning to fermentation and quotes:
Fermentation (Wikipedia) says,
even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to oxidative phosphorylation, as long as sugars are readily available for consumption (a phenomenon known as the Crabtree effect)
Which makes me wonder, "what really causes/encourages yeast to multiply?". If there's plenty of oxygen and sugar, why doesn't yeast just keep multiplying?