(to be honest, I find this "pencil-like" look a bit like MS Comics for fonts, ugly and unprofessional... so I really don't understand why designer tool use it so much)
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.)
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