Why do array type become tuples?

Question

I have a problem with typescript

function watches<T extends unknown[]>(
  sources: [...T],
  cb: (value: T) => any,
){

}

watches([{a:1},'1'],(b)=>{})  // b -> [{a: number}, string]

this is exactly what i want, but why?

why (value: T) => any value is a tuple?

i feel it incredible.