Well, if you're unable to read, you're not going to figure out what the buttons do by reading the textual labels :p
Further, if you have difficulty reading, it's easier to parse the meaning of an abstract symbol, so you'd use that instead of a textual label when available. (I say this as someone who is a really slow reader. I use icons when I can)
(Not the literacy stat but the fact that illiterate people "figure out how to use tech by memorizing the icons and locations of buttons").