What it teaches
How a letter is written, which is not the same as what it looks like. Where the pencil goes down, which way it travels, and how many times it comes off the paper.
Those three things are what handwriting teaching is almost entirely about, and they are invisible in a finished letter. A child who writes o clockwise produces the same mark on the page as a child who writes it anticlockwise, and will be slower at everything that involves a pencil for the next four years.
What to look for
Three levels: small letters, capitals, then the digits. Small letters come first on purpose — they are harder, and they are what nearly everything a child reads is made of. Starting a child on capitals is a habit school spends a year undoing.
It works best on a tablet or a phone with a finger. A mouse is fine and a trackpad is the worst of the three, which is worth knowing before deciding that a child cannot do it.
Three letters finish a round and earn a sticker. Nothing is timed.