Even the most awesome songs become somewhat old and boring after listening to them for the 100th time. Scratching, re-mixing or mashing them up adds something new that makes them interesting again.
DJ Cut Killer mixes movie soundtracks:
Sometimes it can be very subtle and you are surprised when you finally realize what song you have been listening to:
Some DJs are incredibly creative like Skratch Bastid who uses parts of two songs to create the Star Wars theme: