Okay, imagine you created this robot. It cooks, it cleans, it plays video games. It's the best robot ever. However, when you created it, you didn't put any logic in it so that you could order it to do things, and when you turned it on, it decided to go become a member of a reggae band in Jamaica.
Without a constructor, your code is off in Jamaica, unreachable. It is no use to you because you can't call it, you can't instantiate it, you can't even tell it exists. When you call a constructor, you finally create a modifiable instance of your class, and that's when you can use it and the methods it creates. Obviously, this is not a perfect analogy, there are exceptions to this, but that is the gist of it.