The importance of choosing what is in the frame when people take a photograph is to make a relationship between the elements that are outside and inside of the frame to create a context because sometimes the context of the photo is not catch by the lens of camera because it is limited. As a result, photograper need to be creative to get photos whit context and sense.
On the other hand, the importance of choosing what is not in the frame is to make a good composition between the elements through the balance, creativity among other factors. Also, is very important to have a good perspective because search diverse angles will be helpful to get incredible results. Moreover, light is another significant tool for framing because it could create natural lines.
Only elements that create meaning and add to composition should be in the photo.
Excellent answer