Why use custom sounds?

Minecraft is a lighthearted game. It doesn't have a lot of "scary sounds", and what was scary at one point in the game's development is so familiar it doesn't rustle any jimmies. Players don't bat an eye to ghasts or endermen- at best, someone will tense up.

But when you throw in sounds unique only to the adventure you're in, you toy with expectations, you can surprise players. Jump scares, cheap as they are, depend just as much on sound as they do on visuals- the loud sound acts as a cue. "Be scared" says the screech of a Grunt in Amnesia: The Dark Descent.

Audio in a game is feedback, really. It puts emotion and atmosphere into something where it wasn't before.

That clang is called a "stinger". In horror movies, using a piano stinger is a common way to suggest something scary is happening. If you tie it with the appearance of a monster, or turn the lights off in a room, it becomes the sinister Effect to a sinister Cause.

I'm just not able to do that in Minecraft with the existing sounds, so I replace them.