Question: What does mixing really do to a song?
Answer: It gives a song depth and space. It makes a song sound three-dimensional. In laymen's terms, instead of a song sounding flat like a pancake, it sounds fatter like a bacon double-cheeseburger. So many elements go into a mix process that the average listener would never think about (i.e.EQ, FX, panning of the instruments, sampling, audio clips, sound effects, etc). Decisions have to made as well (i.e. What effects should I put on the drums? Should I put reverb, delay or both on the vocal? How much EQ'ing does this track need? Do I put a sample here? What sound effect would be cool to add to this song: a car-crash, the sound of glass breaking or a baby crying?) Mixing is what separates a OK song from a sonically-pleasing 3D experience.
Question: Why is editing necessary?
Answer: Editing is necessary to 1) clean up a track by removing unwanted noises, sound effects or negative words; 2) remove unnecessary or unwanted passages from a song; 3) Shorten a song or manipulate it in such a way to fit a video trailer or custom-made video project. In a nutshell, editing is taking the best parts of a song and putting them together to form a smooth, cohesive track