Copyrights protect the mark as is, and derivatives of it, but does not protect the brand from look a likes or from new trademarks that would be confusing.
Consider the Pepsi logo:
I could easily draw a circle half red and half blue with a white line straight across, vertically, or diagonally. In fact:
There it is my new logo for my cola company called Hep-C (I'm raising awareness). Under copyright law Pepsi could not come after me because the logo is my own creation. I could even call it Pepsi and it would not be a copyright violation as you cannot copyright words. However, Pepsi has better lawyers than that and with my trademark so similar to theirs and the name sounding so similar (and my new name being potentially disparaging to Pepsi) it would likely confuse consumers between what was Pepsi and what was Hep-C thus under trademark law it would be disallowed for commerce.
Now you would have trademark protection by virtue of using your mark in commerce, but it would only be for the states in which you used it. Additionally a lack of a registered trademark would disallow you from seeking statutory damages under 15 U.S.C. § 1117.