It makes it clear when you are generating new symbol, and when you are reusing existing symbol.

    var a = new Symbol("foo");
    var b = new Symbol("foo");
    map[a]="firstValue";
    map[b]="secondValue";
    Console.log(map[a] + " " + map[b]);
    // writes "firstValue secondValue"

    map[#foo]="firstValue";
    map[#foo]="secondValue";
    Console.log(map[#foo]); // what is written here?

Nobody because it's completely daft, and absolutely not equivalent to the second version, the equivalent to the second version is:

    map.add("foo", "bar")