Habit forming
It makes for a fun screen saver and is somehow very habit forming.
My little sister is only five, and she loves going back to the "pet shop" to trade fish in for different fish and then admires the tank's new look. Thanks to her, I now know that when you come back from the pet shop, the new fish are always responsive to food, even if your previous fish were just fed.
On a practical note, the app is not a data gobbler, but it is a battery hog. Don't leave it open overnight if your device is not charging.