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.