Why is GPL better than BSD for example. I have the full source code of Redis and I can do any change I like. Why is copyleft per se a better solution?
GNU & GPL really are about freedom as in freedom of speech. They ensure items cannot get less free when redistributed. BSD etc are more about getting out of your way, so you can do almost anything you want on redistribution.
Copyleft matters to those who care about freedom first. The FSF has a good explanation: https://www.gnu.org/philosophy/free-sw.html
Historically RMS has been seen as a bit of an alarmist over software related freedoms. However the passage of time often shows he isn't alarmist enough! Here is a good starter article from almost 20 years ago http://www.gnu.org/philosophy/right-to-read.en.html