Body lines are pressed into the steel from the manufacture to add shapes, features, and to provide additional strength to the panel. The stamped body lines create tention on the steel, which work hardens the metal. Therefore, the body lines are the strongest points of the panel. It takes more impact to cause damage on body lines;however, once damaged, it will make the repair more difficult as well.
With that said, metal on a body lines or any other part of the panel may be streched during a collision. The streched metal must be shrunk to its original (pre-accidental) state. If not, the metal will be weaker than it should.