The importance of what is and not in a frame is a very bid deal. Objects in the frame could make for a very good photo especially for different angles of view that you could try out and get a whole new perspective on things in the frame. I say the importance really is to get the right angle you think is right for you and see what works in that frame and just have a fun time with it. Real life frames are practically every where and using them in your advantage makes taking photos pretty unique especially playing angles using those frames as well. The frame is everything when it comes to photography and using it for anything could really bring a object or person to life on the photo.
The frame of the camera is used to eliminate distracting elements. Composition organizes the elements to create visual interest and meaning. Real life frames is a technique for emphasizing the subject of the photo