Suppose I want to start an app to run in background as soon as an event occurs in the phone.
For example:-
When I open my gallery I may want to automatically start my app which monitors my file browsing and prepares database for most recent documents, most listened song, etc and is automatically closed when I exit my gallery folder.
Whenever I open my internet browser I might want to start my app in background, which monitors how many times I visited a particular site and it automatically closes when I close the browser.