I'm not talking about preinstalled apps here I mean apps that you buy or get for free (if you are willing to put up with the constant advertisement). There are tons out there that do a multitude of things so which ones do you find yourself using over and over again because they fulfil some need you have and what is it exactly that they do?
One of my favorites is a game called Shadowmatic, you could spend hours playing it if you wanted but it is great as a time passer if you are on a bus for 15 or 20 minutes. Basically you are presented with a randomly shaped floating object and it projects a shadow on a wall and you have to rotate it in different directions to try get the shadow to look like a common object but you're not told what the object is and as you progress the shapes get more and more complex, like at the start you only have to work with one floating shape but a little bit into the game you are given two that rotate separately and if you rotate them correctly their shadows combine to make one whole object. It is honestly so calming to play and the background music is astoundingly relaxing too. If you have high blood pressure it is totally a game you want to invest $1.99 in
This isn't just about game apps by the way, that's just the app I use the most because it helps me chill out and passes time for me if I have a few minutes to spare. Your example can be any type of app