Why do we need an interface?

In Model-View-Controller terms, the Activity is the Controller of the Fragments. The Activity is responsible for choosing which Fragment to display where, while the Fragment is responsible for what's inside itself. This way they keep their responsibilities separate. For the Activity to know it has to replace one Fragment with another, it has to have a callback implemented to do so.

Interfaces help with reusing Fragments. If we directly change to a new Fragment in the Fragment itself, we're hard-coding it to work directly with one particular Activity class and one particular new Fragment class. Using an interface means we don't have to code what specific actions to take. We simply send a message that an item was clicked and let the callback handle the details. Then any Activity that implements that interface will be able to use that Fragment instead of just one. (This is called loose-coupling vs. tight-coupling.)