They are often color coded for sound, letter combination or spelling. Once they are laminated, children can write on them in dry erase marker. I have them circle letters or vowel combinations, or trace them. I've designed a couple of laminated books that demonstrate sound-spelling relationships that I use in the same way. Once finished, the book wipes clean for the next child.
Nursery rhymes demonstrate the variation in words that sound nearly alike.
The repetition helps the child rehearse and recall words. The cards I make often have pictures or symbols on the back, but never a picture next to a word. Pictures tend to confuse what you are trying to teach. You want to make the connection between the sound of the word and how it looks in print. Word meanings come easily enough with the sound.