Individuals can survive in some areas of modern life without learning to read; in fact, the value of universal, public literacy has only been widespread for the past 800 years or so, beginning with the end of the Middle Ages. Acceptance of illiteracy continued through the modern slave trade, when literacy was viewed as dangerous for slaves to have, a possible catalyst for organization and rebellion.
Today, one way in which illiteracy disadvantages people is by limiting the rate of learning. People who cannot read depend on the individuals they meet in person in order to acquire new knowledge through spoken discourse. In contrast, people who can read generally have access to libraries, search engines, encyclopedias, and other curated storage systems of knowledge.
Another way in which illiteracy impacts people is by inhibiting them from experiencing written stories that can help develop empathy. Written language exposes readers to the internal lives of others, demonstrating how they think differently yet arrive at analogous conclusions, values, and stories that expand one's own conception of what it means to be human. In an increasingly globalizing world, where people meet other people they would not have been able to meet even a generation ago, empathy is more vital than ever.