Why is yeast used in bread making? This question arose today at lunch, and we tried to explain it in various ways:


It gives the soft (spongy) consistency;
It improves the nutritional content;
It makes bread more digestible;
It improves the taste.


We were unable to say which ones are true, and if more than one, which is the main reason. It should be something simple I guess, because yeast has been used since ancient times.

Wiki talks about consistency, but I would like some more expert advice.

EDIT: I know there are yeast-free breads, like Italian's piadina or Indian's chapati and roti, and even Mexican's tortilla. And that's where the question originates from, because the asker is from Pakistan.
There are various types of bread without yeast. Unleavened Jewish Passover bread, for one, and soda bread for another. Unleavened bread doesn't have the spongy texture.

Basically, yeast has two uses in baking. 1. The spongelike structure and 2. Taste.