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?
Because, and I say this with all humility, everyone of us concedes (or at least should concede), we could be wrong. 

Besides- one thing we know about God's Truth is that it can always stand up to rigorous inquiry. If it did not, either we serve a weak God (which we don't) or our understanding of God is wrong.

Either way, the last thing any Christian should be scared of is the Truth.