When something becomes so easy to do it's common to say "I could do this in my sleep". But have you ever tried to do something super easy in your sleep.
For some reason our mind work in a mysterious ways while we sleep. Most of it doesn't make much sense, but there are those moments of clarity when the dream feels like real. Until you try to do something normal and it goes horribly wrong. It's a good way checking whether you are sleeping by just doing something common. If it doesn't work as intended you are most probably sound asleep.
Despite all the studies there isn't that much facts about all the mechanisms that go through while we sleep. Theories suggest the dreams are a sort of cleanup mechanism sorting out our thoughts and memories or that they prepare us to face the hard times (e.g. nightmares). But what's the point then dreaming about something we already know well, and especially dreaming about failing on it?