Why do some Christian churches have classes about other churches?

The context of my question is is directed mostly toward "non-denominational" or independent Christian groups, though feedback from the perspective of more established (e.g. Protestant) churches is welcome where applicable. I'm aware that there are Sunday-schools by churches to teach their congregations about the scriptures, the gospel, etc. Some churches also teach classes about other churches. Why do some Christian churches have classes about other churches? What is the Christian, not secular, reason for having them?