Koreans use it when they don't understand something or want to know the reason.
Yes. "왜요? (waeyo?)" is polite and commonly used. You can also say "왜?" (wae?) casually with friends.
You can say "Why is that?" — 그게 왜요? (geu-ge waeyo?).
왜요? means "Why?" Polite question. 왜 (wae) = why, 요 (yo) = polite ending. One of the most frequent question words in Korea. Used when you don't understand reason, are curious, or hear unexpected news.
1. Don't understand reason: Ask for explanation.
2. Unexpected news: Curious.
3. Daily conversation: Smooth communication.
Why? What's wrong?
Why? I don't understand. (Formal)
Why? Suddenly? (Casual)
Why is that?
• 어떻게요? (eotteokeyo?) - How?
• 무슨 일이에요? (museun irieyo?) - What happened?
• 왜 그래요? (wae geuraeyo?) - Why are you like that?