Answer:
This is a bit complex, but the reason why we add yeast to bread is because yeast does a process called cellular respiration.
Cellular Respiration is the process of energy being produced in organisms with the consumption of glucose, generally letting out a by-product of carbon dioxide.
A process called blooming usually is needed before the yeast is added, where sugar, water, and yeast are combined so the yeast can do its work.
A short answer is, yeast allows the dough rise and double in size because the sugar or glucose within the dough is used by the yeast to create the carbon dioxide gas allowing the dough to rise and make bread.