They look cool and animalistic, at least the naturalistic cats'-eye sort do. Glowing eyes look like that to humans because pretty much every other vertebrate on the planet has a reflective layer stuck on the back of their retina which gives the photoreceptor cells another go at picking up incoming light. Great for night vision, bit blurry for daytime stuff.

Some instances of this are done by magic or sci fi, which means they can look like anything you like of course. Stuff the proper science, this is about special effects. Glowing sclera on their own look awful though: artists, don't do that please.

See also

• Tapetum lucidum @ wikipedia
• Tarsier @ wikipedia - a nocturnal thing descended from a diurnal critter, poorly.

• eye_mist
• glistening_eyes
• star_oculama for the anime trope of a character's eyes flashing in a four-pointed star pattern