As for the main questions: Yes, yes, yes, sometimes, not really.
Flasks are kind of a hipster thing anymore, but if you ever look at a half-pint, or 200ml booze bottle, it's basically a clear flask. The reason being is that the shape fits nicely in one's pocket, or is easy to tuck away in the band of stockings, so it's convenient method for carrying booze. The other advantage a flask has is that it's easy to conceal, this is generally what causes issues with the image of flasks, it's something a drunkard would likely carry to sneak some liquor. Flasks are seen as kinda cool though because they let you carry around some booze for an impromptu drink, which seems to be really cool with hipsters.
Really what they are, is something useful for someone who does drink. I have one just in case I want to sneak some liquor into a movie.
A quick hint to anyone who wants a flask, all metal ones are really shitty, they tend to corrode on the inside, even the aluminum ones. So you want to find a high quality glass lined one, or a glass one with a metal case around it, because glass doesn't corrode. Though any flask that is glass lined will break inside if you drop it on a hard surface.