In baking, what is the purpose of yeast?

The purpose of yeast in baking revolves around its ability to ferment sugars. When yeast is added to dough, it consumes the sugars present in the mixture, producing carbon dioxide and alcohol as byproducts. This fermentation process is critical because the carbon dioxide gas gets trapped in the dough, causing it to rise and develop a light, airy texture. This leavening action is fundamental to many baked goods, such as bread and rolls, which rely on yeast for their characteristic volume and structure.

While yeast does play a role in influencing the flavor of the dough through fermentation (which could be tangy or complex), its primary function in the context of this question is to ferment the sugars and enable the rising process. Without the fermentation that yeast provides, baked products would be dense and heavy.