Anybody who's ever been in a few meetings that try to put together stakeholders, designers, and developers, know how it will inevitably descend in painful back and forth about a shade of hue or an icon size. People get distracted by colors and graphics, and fail to provide actual feedback on functionality and layouting - which are the hardest bits to change later.

The point of this style is to communicate that it's a rough draft, so that people focus on the essential implementation and functionality requirements, the hard stuff. It's easy to give it a lick of paint later. (It also keeps expectations low, so that the final result will feel like you're overdelivering. But that's just bonus.)

For those "downvoting" this comment, please: I wrote it right after my initial post, before any answer, to make my initial post clearer. I certainly should have added this to the main post.

Now that I have all these answers, I understand better. But cant delete or modify this comment. So sadly it's here for eternity :-(

Thanks a lot for your insightfull comments to the original post. Actually, I now think that I will use these method to help getting more feedback from users