What it teaches
Finding the number that is missing from a sum, wherever it sits. A child who can do 7 + 5 often stalls on 7 + ? = 12, because the second one cannot be answered by working left to right. It has to be turned around.
That turning around is the beginning of algebra, several years before the word is used. It is also what makes subtraction make sense: 12 − 7 is the question "what do I add to 7 to reach 12?" written a different way.
What to look for
If a child can do Adding and taking away to 20 easily but struggles here, nothing is wrong — this is the harder task, and it is the one that matters more. Reading it aloud as "seven and how many more makes twelve" is usually enough to unlock it.
Expect it to be slow at first even for a child who is quick at ordinary sums.