What are these hooks on the A320's wing used for?
In the event of a water evacuation, there is a line associated with each overwing exit that is extended, and clips to that yellow bracket, as a guide to get everyone out on the wing. 

From Wikipedia, these are for overwing exits.


  The use of overwing exits in a ditching varies from airline to
  airline. On aircraft fitted with overwing exits, there is typically a
  raised escape rope bracket (about a third of the way from the door)
  attached to the wing's upper surface and typically painted yellow.
  This is accompanied by an escape rope found in the frame of the exit
  after opening the hatch. Once this rope is attached to the escape rope
  bracket, it will aid passenger evacuation onto the wing to await
  rescue or to enter the water (depending on the airline's procedures).
  On aircraft with life rafts to be launched via the overwing exit, the
  wing will be used to provide a boarding platform onto the life raft.
  On certain regional aircraft, the overwing exits are the only escape
  route in the event of a ditching.