I went through a dance phase in college, during which I took a lot of dance classes, including ballet, and as a result, had to wear tights. The women in class had to wear tights, too. I don't think anyone liked wearing tights, and they weren't flattering, no matter how slender and muscular your legs were from dance classes.
So I've always thought the leggings fad was strange. Essentially, you're wearing black tights in public; but while in the past, women would wear skirts with tights, they have now dispensed with the skirts and are sporting the tights by themselves. Why?
The issue came up recently when two girls in leggings were kicked off a United Airlines flight. It turned out the girls were using free passes provided by an airline employee, and users of those passes have to follow a dress code. Regular paying customers can fly in tights only, if they like, and probably in really tight wife-beaters, too. Why they would want to is an unanswered question.
I've heard people say leggings are comfortable, but there's more to it, I think. Is it a statement of liberation — "This is my body revealed in detail in these tights and there's nothing wrong with it?" Or is it meant to be alluring? Or is it a vagary of fashion that started somehow (by accident?) and caught on? I don't know. But just as I never got used to seeing young women in pajamas in the grocery store during that fad, I have not adjusted to this one. They're still tights to me, and they still seem out of place outside of a dance studio.