keeping your transmission cool will help protect seals and ensure years of trouble-free operation.
Transmissions, by their nature, create immense amounts of friction. The best way to combat this friction is to use the oil to help with lubricity and keep heat down. While it does a good job, it can still overheat as the transmission works harder. This caused heat in turn decreases the oils lubricity, causing it to not do as good of a job, and causing faster wear on transmission clutches.