constitutional questment i: wherefore section 2?

I've been reading over the Constitution, and have come across some points that I have questions and comments (i.e., "questments") about. Rather than try to post a list of them, I'll do them one at a time. That way if I tyhink of more, I can post them too, without it looking like an addendum. So here goes with "Constitutional Questments."

The fifteenth amendment establishes that voting rights will not be denied on the basis of race, color or previous condition of servitude. That's in Section 1. Section 2 reads "The Congress shall have power to enforce this article by appropriate legislation."

What's the purpose of the second clause? The only explanation I can think of is a concern that a state could limit voting rights, and then argue that the Federal government can't come in and enforce the rule stated in Section 1. In that event the amendment would be toothless.

Is that it? Is there another reason?