People manipulate images to make them selves look better. Usually people get rid of wrinkles and add more colour to their skin when doing these transformations. Sometimes people even enlarge or shrinking certain body parts to make them selves in their pictures look appealing to them or to others.
But celebrities do it to keep up appearances for example the George Cloony picture on the left. The one on the left is unedited and the one on the right is. You can see the edited photo had wrinkles air brushed out and George Cloony skin and hair had been darken to add colour.
Another reason celebrates or advertisers edit photos is to sell a product. The picture of Keira Knightley on the right shows a poster of her in King Arthur. The left half is unedited and the right half is edited. The edited poster has added colour and Keira Knightley chest has been enhanced.
To edit photo in photoshop, people use many tools like air brush and burn tool; which hide and fill stuff people do not want other people to see.