When you are in a public place and witness a marriage proposal, I'm sure that would send fluffy butterflies tingling all around your body. Personally, I have never seen one in my life; but I could imagine witnessing one. I've seen tons of videos on Youtube on proposals gone right.
I think that apart from being romantic, some guys do propose publicly so the girl won't have the courage to say "no".
But in any case, what if she said NO?
That, for me, would be the most cringe-worthy moment ever.