Got you, suckaz!
Good! I'm glad someone posted what immediately came to my mind. Thank you!
or someone who uses Ubuntu because he's scared of the command line
Heh, well put!
Spoken like someone who doesn't develop software for a living.
I develop software for a living and I don't require the restrictions of copyright to do it. It's all used internally to support our research.
You're getting lots of answers, but they're all just circular: "Because the license says so." That's an uninteresting answer that misses the purpose of your interesting question.
Imagine the time when the GPL was conceived. There was no world wide web, and the Internet still barely existed. RMS was selling copies of Emacs source code over snail mail on tapes. At this time there's not really a central, official repository for the various projects going on. It's just people passing source code around to each other in a distributed fashion, with the GPL enforcing the distributed system. That way there's really no authority for the code. Someone gets it, modifies it, passes it along, and so on, forking and branching like modern distributed version control. The distributed nature of the code makes its availability more robust.
It's still important today because it's better to have code distributed from many places than from a lone host, which would be a single point-of-failure.
It is easier to write an incorrect program than understand a correct one.