Why do some Catholic Cathedrals have gargoyles on them?

I'm Catholic, and I just came across a video on youtube mentioning that there are gargoyles on Catholic Churches. Now that I think about it, the cathedrals in Europe have them. I'm wondering why? I mean, I think I heard that they are used to ward of evil spirits, but gargoyles are evil symbols. You can't ward off evil with evil. I don't mind a St. Michael statue with him overcoming a devil, I rather think that it's a cool image, but gargoyles are just like hanging out on a church building. Just wondering about that, I'm sure there is a good explanation. I really want to hear from devout Catholics who know about this subject matter. I know there will be Protestants who will answer and say because the Catholic church is anti-Christ, hoar of Babylon, etc... I've heard it all before. If you are going to say something like that, I'd rather you not answer. Thanks and God bless.