Yu Yevon and his followers were sick of the constant warfare between the city-states of Bevelle and Zanarkand. So they created a mystical babysitter for civilization: Sin.

Sin's purpose is not necessarily population control, but to keep large technological centers from being built, to limit the construction of new technologies (which ultimately would be used in warfare). Also Sin was Yu Yevon's revenge on the people of Bevelle. Now, to survive the rival city-state would have to worship him, and continually sacrifice their own people to him to avoid a one-way ticket to the far-plane.

Sin needed to be a being tortured between commands of Yu-Yevon and the love of its own people, its own loved ones. The whole journey of the summoner was meant to create strong emotional bonds, and to give them a fatalistic view of Spira. Sin, Yevon, Aeons, all of these things are apart of psychological warfare constructed within the mind of a being who is essentially immortal.