Why does a compressed spring have a "capacity for doing work"? Why does it possess energy?

To compress a spring we apply force on it. This force is stored in the spring as potential energy. Once, the spring is released, the potential energy is converted into kinetic energy and comes back to normal position. When a material is moved through a distance by a force work is said to be done.

Thus the compressed spring has the capacity for doing work.

Energy possessed by a spring is called 'Spring Force'.

Spring Force, F =k.x     
  where, k=spring constant
              x= distance moved while compression of the spring(amt of compression)