Why do I need to accept permissions when using an application?

In Android™ 6.0 or higher there is a permission model called Run Time Permissions, where an app prompts you to grant permissions while running, not when you install it. The purpose of the new model is to give transparency to what permissions an app needs. When you are using Snapchat™ to take a picture, for example, the app does not have access to the camera and will prompt you for access. App permission prompts only occur once if you accept. However, permissions can be modified at any time by going to the app's Settings screen.

To adjust permissions

1. From your Home screen, tap the Application screen icon.
2. Find and tap Settings > Apps.
3. Find and tap the app.
4. Tap Permissions.
5. Drag the sliders to grant or deny permissions.
6. Tap Additional permissions. 
7. Drag the sliders to grant or deny permissions.