I believe that the % goes up based on the actual age, 0% bonus in age of exploration, 50% in age of reformation, 100% (total) Age of absolutism and 150% (total) Age of revolutions. In many cases without that bonus it may be dificult to get a good amount of banners compared to your development unless you mass converted to Manchu, whereas with the bonuses even with only started Manchu land you will be able to get a good amount.