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.)