When beside launch will android.intent.action.MAIN be fired by Android

If an Activity has the following filter:

 <intent-filter>
         <action android:name="android.intent.action.MAIN" />  
         <category android:name="android.intent.category.LAUNCHER" />
 </intent-filter> 

Under what circumstances will an Intent matching the above filter be issued by Android?

1) During launch of the application.

2) ? Are there any other circumstances?

Anything can use that filter. The OS never uses that filter, AFAIK. Home screen applications typically use that filter. Other applications can use that filter if they so choose -- for example, here is a sample app that creates its own launcher-style list.