Imagine your computer breaks down. It doesn't power on anymore. Now imagine you're not allowed to open it and the only person who may fix it would be the producer. It would probably take weeks for you to send it in, get the machine fixed. That's if the error was found and it got fixed. And then it might end up being pretty expensive. Because there's a monopoly on the repairs.
That's exactly how closed source software works. And that's exactly why you should prefer open source software. The more important the software, the more important that it's open source.