Sometimes when working on software the goal is not to get a competitive advantage, but to promote some ideas. Copyleft license is a tool that aims to help with

Sometimes when working on software the goal is not to get a competitive advantage, but to promote some ideas. Copyleft license is a tool that aims to help with granting, that the work derivatives are available to the public to read and modify (i.e to prevent closing the source code of the program that is commercially sold). The concrete difference you ask about is that the work derived from copyleft code retains the license and the source code can't be closed. If you scrap the license, then the code created by someone who had clear goal in mind when writing it for not making improvements over it closed source, ends up with possibility of being closed source.