The purpose is to reduce eddy currents in the core while keeping a high magnetic flux.
[2] The stacking factor depends on the thickness of the lamination of the steel sheets which comprise the core.
Stacking factors are typically 0.95 or higher for transformer cores[4] and machine stators.
[5] However, cores made from amorphous metal have a stacking factor of around 0.8, compared to 0.96 for silicon steel.
The higher the operational voltage of the transformer, the smaller this ratio needs to be in order to provide more space for insulation.