Because there are millions of people out there in the world who can't read and write.
From a historical perspective, literacy levels for the world population have risen drastically in the last couple of centuries. While only 12% of the people in the world could read and write in 1820, today the share has reversed: only 14% of the world population, in 2016, remained illiterate. Over the last 65 years the global literacy rate increased by 4% every 5 years – from 42% in 1960 to 86% in 2015.