There are a couple of reasons to breed villagers in Minecraft. Firstly, you can trade with them. As every villager has a different profession, you’d want to have enough villagers to ensure a supply of all necessary goods. Secondly, your villagers can die for various reasons, and you have to replace them. Thirdly, growing your village is simply fun, and when the village is large enough, iron golems spawn automatically to protect the inhabitants.