What it teaches
Opposite pairs, which are the easiest vocabulary of all to learn because each word carries the other one with it. A child who learns heavy alone has one word; a child who learns heavy and light has two and a relationship.
What to look for
Some pairs are not really opposites and a child will notice. The opposite of wet is dry, but what is the opposite of blue? Noticing that not everything has one is worth as much as the pairs themselves.