Set and spread operator

Question

Why is it necessary to use the spread operator with a set why can't I log it to the console without using the spread operator?

This wouldn't work, what is going on to make this not work?:

function duplicateSet(str) {
    const arr = str.split(" ");

    const set = new Set(arr);
    const newString = set.join(" ");


    return newString;
}

console.log(duplicateSet("This is is a test test string"));

This works but why?:

function duplicateSet(str) {
    const arr = str.split(" ");

    const set = new Set(arr);
    const newString = [...set].join(" ");


    return newString;
}

console.log(duplicateSet("This is is a test test string"));