It's called "sight reading" not as opposed to reading-without-seeing, but to indicate that you can read and decipher the notation immediately upon seeing it, and can sing or play as you read ~ as opposed to scratching your head and figuring things out so that you can eventually sing or play the notated music after a period of study and practice.

Many of who are unable to truly, literally "sight-read" are able to use musical notation "as a reminder" of just how to sing/play a piece with which we are basically familar but that we do not have down cold. Like glancing at the notes in a hymnal while singing along with the choir...