[rpastuszak]

A) Make it easier to focus on the core aspects of the problems instead of obsessing with details (applies to both designers and "reviewers")

B) An "unfinished" messy design is an invitation for critical feedback. If you give people something that looks too polished, they might be afraid that they'll break it, that they don't understand it, that they can't give feedback that is "good enough".

In short: if it looks like a toy people will play with it.

* C) The reason many of these tools look like Balsamiq has more to do with the tech of the late 00s/early 10s. This specific style of vector art was pretty easy to achieve in Flash.