In the "safe" MS Windows desktop application development land, our application has a lifecycle, but it is not crucial to take care of it. Usually, you have a set of events to handle such as
FormClose (at the form level), or
Application OnRestore, or
Application OnTerminate. In some cases, you have to handle the state where the main application window is minimized, and this is still simple. In the mobile world, as usual, things are a bit more complex. The concept of lifecycle is evidence. Just to make things messier, an Android activity's lifecycle is different from an iOS view lifecycle. Remember, when an app is in the background, it can be completely destroyed.
But, hey! Why I should ...