What is the use case for declaration merging?

I'm reading a bit about declaration merging in TypeScript and I have a hard time grokking the use case for it, specifically for interfaces.

In their documentation, they have this example:

That is, in the example:

interface Cloner {
    clone(animal: Animal): Animal;
}

interface Cloner {
    clone(animal: Sheep): Sheep;
}

interface Cloner {
    clone(animal: Dog): Dog;
    clone(animal: Cat): Cat;
}

The three interfaces will merge to create a single declaration as so:

interface Cloner {
    clone(animal: Dog): Dog;
    clone(animal: Cat): Cat;
    clone(animal: Sheep): Sheep;
    clone(animal: Animal): Animal;
}

Why would one want to create three separate interfaces instead of the resulting declaration?