It's just that as a user, I'm always happier when I get access to the sources of the product I buy. So I release my code under a copyleft licence for the sake of the other users like me.
To push the idea to the extreme, imagine a world where all the open source software that was ever written was under the GPLv3? Maybe most software would be open source in one way or another (and GPLv3 gives you a way to update the software). So you could buy a smart TV, get access to its sources and to tools that would allow you to upgrade it. I think it would be pretty amazing.