Sight Words are memorized so that a child can recognize commonly used or phonetically irregular words at a glance, without needing to go letter-by-letter.
Knowing common, or high frequency, words by sight makes reading easier and faster, because the reader does not need to stop to try and sound out each individual word, letter by letter. Learning sight words allows a child to recognize these words at a glance - on sight - without needing to break the words down into their individual letters and is the way strong readers recognize most words. Sight words are words that should be memorized to help a child learn to read and write.