How would the GPL work if there were no copyright on software?
For example, lets say you make a peice of software called TextWriter. I like it, so I take it and modify it, add some stuff and make it incomputable with your version but still use most of the same code and a similar GUI that you developed. I don't give anyone the source code, or anything else but a $100 binary executable. If there is no copyright on the software, you can say you publish it under the GPL all day and I don't have to listen to you, because you have no legal right to control what I do with the software.