Why do superheroes wear underwear over tights?

One of the most common answers: 

spandex so tightens the body, that if you do not add to the suit some sort of mini breeches, it seems that the hero is almost naked.

In fact, everything is not so simple.
The fashion for wearing underwear over tights was set by Superman.

Some insist that Superman wearing not an underwear, but the armor. This was the element of knightly armor, an armored triangle, for the protection of the genitals. Subsequently, it's transformed, and became the decoration of the costume.

Superman quickly became phenomenally popular and set the fashion for superheroes. Publishers demanded "something in the same spirit," and the artists obediently copied the alien from Krypton.

Tight outfits got accustomed also because they are easier to draw than complex detailed suits.

For decades, superheroes have been worn underwear-over-pants, but in recent years their outfits are constantly trying to modernize.

In 2011, even Superman in Action Comics started wearing jeans.

So, it is likely, soon people will stop asking: "Why superheroes wearing underwear on top of the tights?"