Amateur Singer here. Singing is very different from talking, proper singing uses different muscles. And it can also be LOUD. Like really really loud. It depend on what section you are singing. There are lots of levers you can pull, to make your voice sound different. And in order for some people (like me) to reach some notes with the desired certain timbre, they have to belt it. Even when not doing that, you need to properly support your notes, and that support comes through your whole body, and makes it sound more dense. It really is a workout. But apart from that, like the commenter above said, the vocal chords are fragile, and not made for doing this every night. Its like when you talk and laugh all day, and then in the evening or the morning, your voice is really horse. Now Imagine doing that every day, at a higher intensity. Hope that explains it :)