Well for one the art is fantastic, but honestly I think the biggest thing visual novels have that other games do not, is that a visual novel can tell a story without the need for bloodshed.
You can tell an emotional, impactfull story, have agency over certain parts of that story, and your character isn't some sociopath fighting his way through hundreds upon thousands of doodz.
Jon Tron was recently on the Co-Optional podcast and described The Last of Us (a game I haven't played, but had a friend who was really hyped for it ultimately drop it due to this) as "A video game pretending to be deep that's just another excuse to blow peoples heads off".
The less agency a player has, the better a story you can tell to the player. The more agency a player has, the more chance there is of the player creating their own story through game play mechanics and emergent game play. Both are valid forms of entertainment and both do unique and cool things for the player.