[8-Ball]

For starters, no-one ever really uses pure metals.

It may be 99.99% pure, but it will still have defects and these defectes will have associated free enrgies and strain fields which will have an effect on electron mobility.

Annealing will provide thermal energy to allow for the diffusion of vacancies and dislocation motion.

This in turn will lead to a coarsening of the microstructure, where by, before it will have been packed full of dislocations and vacancies, and after the smaller number of dislocations will be aranged into low angle grain boundaries, with a much higher proportion of the material having a low defect concentration.

This will improve conductivity.

As a side note, annealing can be and is used to homogenise the composition of materials which have undergone dendritic solidification, or any other solidification mechanism which results in microsegragation, though this is not what we are trying to do here.

8-ball