Mating with multiple males helps the female red jungle fowl (the wild ancestor of the domestic chicken) produce offspring more resistant to diseases.
By favouring the sperm from males that are most genetically different to them, females increase the diversity of Major Histocompatibility Complex (the gene that detects and fights infections) within their offspring.
This effect, however, was lost during artificial inseminations.
Read more at University of Oxford, Stockholm University and Linköping University