Randomly sorting an array

I just watched the Javascript Foundation, Methods: Part 1 video and have a question about sorting arrays.

Here is the code we write:

var my_array = [10, 44, 32, 100, 0, 44, 3, 4];

my_array.sort(function (a, b) {

    return Math.random() - 0.5;
});
console.log(my_array.toString());

I'm still not clear exactly how this sorts the elements of the array randomly, and why we need to return negative values (from -1.5 to 1.5).

I thought the Math.random return assigns random values from 0.0 to 1.0 for all elements in the array. Shouldn't this be enough to randomly sort the array? I understand the .sort method arranges the elements based on alphabetically ordering, not numberic, but shouldn't giving them new random values be enough even if these values are sorted alphabetically?

So I removed "- 0.5" from my code to see what happens and this is what I see in the console:

4,3,44,0,100,32,44,10

It just reversed the ordering of the array. And this is the result I get every time, it isn't random at all. I would like to understand why this happens.