What it teaches
Listening, on its own. Every other vocabulary game shows the word as well as saying it, and a child who can read will read rather than listen. Here there is nothing to read, so the ear has to do the work.
This is the skill that transfers to a real conversation, and it is the one that is usually skipped.
How it works
The word is spoken and pictures appear. A button repeats the word as many times as a child wants — asking to hear something again is a good habit, not a failure, so it costs nothing.
A wrong picture keeps the question. The word is spoken again before the answers are offered a second time.
What to look for
The voice comes from the browser, so it is the one set on the device. On an iPad the English voices are good; on an old Android they can be poor. It is worth checking, since the whole game rests on it.