Beginner question about these intents and activities...

If I plan to do something with the phone hardware or file system, is it necessary to use an activity for doing so? What is the purpose behind this system?

I somewhat understand what you are asking I think. For example if you have a button, and when it's clicked it writes a file. The act of writing the file doesn't require another activity to be displayed, you would just have a event that fires in the current activity to write the file or whatever you choose to do when you click that button.