Actually, the original writer for the Joker created Harley as his love interest based off of "a friends stormy relationship". Quoting said writer, along with the Harley and Joker's voice actors:
Paul Dini: He loves her as much as he can. He loves her in his way.
Arleen Sorkin: Everyone sees the Joker laugh. Only Harley has ever seen him cry.
Eyes are the most expressive thing in a face, so costumes and art like to make the most of them (or else have reasons for hiding them). And white spaces are simply the easiest way to show eyes against a dark mask.
In various comic media, the masks worn by the Bat Family are Lenses.
It is for the expressions and dramatic effect, just as Luciano said. The white glowing eyes make Batman look mysterious and a bit frightening, just as how you would be scared when an animal's eyes glow in the dark. This is probably why Bruce does not have glowing eyes and you can see his pupils, to show that he is a normal person irl.
Many (most?) of the masked superheroes in comic books have white eyes. There are 2 probable reasons:
dramatic effect: as the rest of the mask is darker, drawing the eyes entirely white make them more visible, and easier to convey expressions. You don't need to see the rest of the face in your example picture to know Batman has an angry expression.