Copyright gives the people that create various works, certain legal controls over those works. As the name suggests, it limits copying (thus various forms of usage) to those authorized to do so. Depending on jurisdictions, it also codifies things such as moral rights.
The Creative Commons licences simplify an author’s ability to authorize copying and use of their work. CC licences leverage the control that copyright establishes and an author can use these licences to, in a sense, automate control. Rather than negotiate requests from every party that wants to use, derive new works, or copy the work, an author can clearly state what they’d like to be able to happen with the work upon expressing it to the public. Then anyone can use it as the author has intended.
Because the CC licenses are not just written with legal use in mind but also for a layperson or machine, it’s relatively easy for creators to take advantage of these licences without entering into a more complicated process.