On Android, you'll often find that you want to send your user to another Activity to do something, and then return them to your current Activity with the result of that action. Good examples are having the user pick a contact, or take a photo with the camera app. In these cases, Android uses a system of special events that are built into the Activity class. For capturing travel expense claims, your user needs to be able to go select a file to attach things such as photos or email attachments to their claim.
In order to present them with a familiar file chooser (and avoid writing a file chooser ourselves), you'll need to use this mechanism. However, to read files from outside of your application's private space, you'll need it to ask the user for permissions. Anytime an application needs access to potentially sensitive data...
