In 15th century men married more than one wife to have more kids because child survival ratio in those times is 20-30%. That means if some women gives birth to 10 children 3 will live to adulthood. This was true of kings also who had superior health care, food and nice living conditions. Further kids died during frequent travel, war attacks, assasins sent by enemies and relatives etc for royals. The Mughals were basically nomadic royals and travelled from one fort to another with family and court. Hence apart from political reason and love the kings married many wives so they have many heirs
Ex: Take case of mughals
Jehangir had 15 kids only 7 lived to 20 and above. Finally when Jehangir died at age of 58 in 1627 only one son Shah Jahan and 2 daughter lived out of 7.
Shah Jahan 14 kids from mumtaz 7 lived to adult life and many more from first 2 wives but 2 girls survived
Akbar had more than 15 kids(exact count not known to anyone but above 15 if you count from all listed) and 3 sons and 3 daughters survived above 30 years of age
Humayun also had many kids but 2 sons and 5 daughters survived adulthood
So those times we can assume looking at this mughal emperors children list only the ratio is 30% -40% with all facilities then local commoners you can only think what ratio of child survival will be there. Hence men married many times so they can have many kids from many wives so chance of survival more for kids. This was true of commoners too not only royal, nobels etc
History and Chronicles 