What it teaches
Where the letters live. A child who has to hunt for every key cannot write anything longer than a word, and the hunting — not the spelling — is what stops them.
The layout on screen is the real one, so what is learned here is what is on the keyboard in front of them.
How it works
A letter appears and the child presses it, on screen or on a real keyboard if the device has one. Nothing is timed.
On the gentler settings the right key lights up after a long pause. On Brainiac and Mastermind it does not: those settings are a test, and a test is not marked by the examiner.
What to look for
Do not push for the correct fingers at this age. Finding the keys at all comes first, and touch typing is a separate job for later.