Can you explain to me the difference between returning value, reference to value, and const reference to value?

**Value:**

    Vector2D operator += (const Vector2D&amp; vector)
    {
        this-&gt;x += vector.x;
        this-&gt;y += vector.y;
        return *this;
    }
**Not-const reference:**

    Vector2D&amp; operator += (const Vector2D&amp; vector)
    {
        this-&gt;x += vector.x;
        this-&gt;y += vector.y;
        return *this;
    }
**Const reference:**

    const Vector2D&amp; operator += (const Vector2D&amp; vector)
    {
        this-&gt;x += vector.x;
        this-&gt;y += vector.y;
        return *this;
    }

What is the benefit of this? I understand the sense behind const reference passing to function as you want to make sure not to modify this value on which reference is pointing to inside a the function. But I&#39;m confused by the meaning of returning const reference. Why returning of reference is better than returning of value, and why returning of const reference is better than returning of not-const reference?


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C++ Return value, reference, const reference