Annealing is done to remove many types of strain in the materials. How it affects the properties of the metal depends on the specific alloy.
Strains and stresses can be set up in metal through many different processes. Drilling, casting, welding, soldering, cutting (mechanical or torch), bending and rebending, cold hammering, rapid heating and cooling especially of small sections of the material, and many other processes can set up strains in the metal. Annealing allows the mass of metal to regrow its structure and remove any stesses that have formed.