What it teaches
Comparing quantities, and the signs that say so. Underneath, it is two sums and a judgement, which is more work than it looks and more like the maths a child will meet later.
It also keeps the idea of equality honest. Children who only ever see sums written left to right come to read "=" as "and the answer is". Comparing two expressions puts it back to what it means: the same amount on both sides.
What to look for
Watch for a child answering by the size of the numbers rather than the size of the answers — picking the side with the 9 in it, for instance. It is a common and quite reasonable shortcut, and the game catches it quickly.