What it teaches
Hearing pitch, which is the foundation of everything musical and is not taught by naming notes. A child who cannot hear that one note is higher than another cannot sing in tune or follow a tune, and it is a skill that improves quickly with practice.
What to look for
This one needs sound, so it is not a game for a quiet room. Headphones help considerably, because the small intervals at the higher levels are genuinely hard on a tablet speaker.