Interior vs. Exterior

In the book club meeting today we discussed Harper Lee's famous novel To Kill a Mockingbird. One of the scenes in chapter 12 was about Capulnia helped Jem and Scout dressed up nicely for church. Then I raised a question: Why do people have to dress up nice for church? Arthemesia answered my question. She said, "This is because people want to show their vanities to the God." Well, perhaps she is right, but I still don't understand why is it so important to show vanity to the God. In my country, we care about human interior more than exterior. In other words, humbleness, sincerity, and respect to the God is much more important than how they dress. We do have clothing codes for every religion in my country, for instance, Taoists and Buddhists should wear clean and appropriate dresses in temples, whereas Muslim has its special dress codes for Islamic women, like clothing must cover the entire body and must hang loose so that the shape of the body is not apparent. After all, I do not mean to compare the dress code of every religion because I know every religion has its own rules. I am just currious why people wear what they do.