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.
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.