compositing

Hi All, Very new to compositing. Why do you cut out an object to paste onto a background rather than mask the object on the original photo and add the masked photo to the background. It seems to give the same result with the ability to 'improve' the mask at a later date if needed? What am I missing?

There lies an old statue beheaded and broken...in the forbidden, darkened wood. Where light radiates from unknown depths and purpose...