My take on this is that it is effective because it reduces peoples fear of suggesting an idea.
We all feel something for our own genuine suggestions, a sort of ownership, and so fear the possibility of rejection as it would be a rejection of something personal to us, our preference.
When someone suggests a universally and whole heartily rejected idea we can take a degree of security from that that our idea will not be as universally disliked; "Well it's not as bad as McDonald's"
It's also possible that it just gets the "flow" going, which again is more to do with removing peoples fears i.e. "well, he suggested an idea and it wasn't so bad for him so I may as well suggest one as well"
We all feel something for our own genuine suggestions, a sort of ownership, and so fear the possibility of rejection as it would be a rejection of something personal to us, our preference.
When someone suggests a universally and whole heartily rejected idea we can take a degree of security from that that our idea will not be as universally disliked; "Well it's not as bad as McDonald's"
It's also possible that it just gets the "flow" going, which again is more to do with removing peoples fears i.e. "well, he suggested an idea and it wasn't so bad for him so I may as well suggest one as well"