Simple, kid-friendly explanation (age 10) of special clothes in Christianity: when and why Christians wear vestments, habits, Sunday best, and clothes for ceremonies.
Short answer: Most Christians do not have to wear special clothes all the time. But some people wear special clothes for church services, ceremonies, or because of their job or community.
Some Christians wear special clothes for services, leaders wear special vestments, certain communities wear plain clothes all the time, and many Christians simply wear regular clothes or their Sunday best. It depends on the church and the occasion.
If you want, tell me which church or picture you saw and I can explain the specific clothes you noticed.