All topics must have a frame if they are publishing data from a particular sensor that has a physical location in the real world. For example, a laser is located in a position with respect to the base link of the robot (usually in the middle of the traction wheels in wheeled robots). If we use the laser scans to detect obstacles in the environment or to build a map, we must use the transformation between the laser and the base link. In ROS, stamped messages have frame_id, apart from the timestamp (which is also extremely important when synchronizing different messages). A frame_id gives a name to the frame it belongs to.

However, the frames themselves are meaningless; we need the transformation among them. We already have the tf frame, which usually has the base_link frame as its root (or map if the navigation stack is running). Then, in rqt_rviz, we can see how this and other frames move in respect to each other.