Why the hell they are singing?

I was just as confused as you. I'll be honest though I didn't know it was a musical before I saw it. So that first song was a big shock, but at the same time it felt like a performance that was actually being done in the story, which made it seem a little less weird, not a lot though. So when the other songs came along, they were very confusing because naturally I thought like you; 'why the hell are they singing?'
Overall though, I guess they were a good idea. They gave each character a chance to monologue but without us having to sit and watch someone just talk for a few minutes.

"God, when I meet you, I'm gonna be pretty. If it's the last thing I do. I'll be a beautiful angel."