If you have Darkvision, you can see in Dim Light within a specified range as if it were Bright Light and in Darkness within that range as if it were Dim Light. You discern colors in that Darkness only as shades of gray.

Darkvision makes your eyes glow. Not a lot! But like an animal's eyes in a flashlight. It means that in complete darkness you'll suddenly stand out, and you might be noticed.

The reasoning is twofold. First, it just justifies the common narrative convention of showing the glowing eyes of the monster before it attacks. It can even be a threat display. Secondly, it adds a tactical element to Darkvision. It lets you sneak close while paying attention to conventional light and dark for advantage, then decide when to switch it on. This is why many monsters appear to have glowing eyes when they get angry: it's showing they're ready to fight.

Source: https://forum.rpg.net/index.php?threads/make-darkvision-more-interesting.882243/