Ok, I'm going to attempt to explain why open source software is better then closed source. For the libre-software folks in the crowd I'll be addressing copyleft and the four freedoms in a following post.
I'm going to explain this using an analogy and what I hope is an apt one- that of a recipe. This is something everyone is familiar with and has probably worked with at one point or other.
Source code is a recipe for how to make a program. Depending on the language it is either "baked" (compiled) or "eaten as is" (interpreted).
Like a recipe the source code is just a list of things to use (resources) and instructions on how to use them (the program).
Just as many recipes need to be baked and one can't easily identify what went in to the recipe after baking, many modern programs are compiled and what comes out of the compiler looks way more like cake then eggs, flour, sugar, vanilla, etc. Therefore it is very hard to work on a program after it is compiled. Just imagine trying to add more oil to a cake that came out too dry after it's baked. It just won't end well.
Ok, now that we have the analogy established, imagine a world where recipes were all legally protected secrets. The only food you could buy was pre-cooked or ready-to-eat. Hate the flavour? Too bad. Want to add blueberries? Sorry can't do that, or at least not in a meaningful way. Ovens would be for heating alone just as most people's computers are just for using a browser.
Worse still, if you did figure out how to make a brownie, somehow found the ingredients and tools to use them and then, GASP! used your oven for cooking, you'd probably promptly get sued by the local big brownie concern for stealing their secrets. And because they are secrets you couldn't prove that you didn't or it would be very hard to.
In this world almost no one could help you with your brownies as only a select few know how to cook or what cooking even is, other then "That thing specially trained people do for big companies".
This is the world of closed source. This is the world of the late 80's and early 90's before the open-source movement. There were a few small pools of hobbyists keeping programming for fun alive but mostly all the recipes had disappeared or were very old and stale.
Now imagine a world where everyone publishes their recipes. And because of this the tools to use the recipes are readily available. If you didn't like Magoo's chocolate cake, you could download the recipe and fix it and bake your own. Now depending on the license that Magoo attached to the recipe you may or may not be able to tell anyone about how you fixed it, and may or may not be able to sell the better cake you made. This is where software freedom and copyleft comes in which I'll talk about in a later posting.
In this world there would be lots of people cooking, sharing ideas on how to cook, how to cook better, coming up with new and interesting things. Also people could look over Magoo's recipes and say "Too much salt in cake #3, it should be 1 teaspoon not 1 tablespoon". Also people could make sure Magoo's wasn't including rat poison, or making a frosting of raw eggs, sugar and lard that'd go off in a day and lead to people getting sick and dying, thus making everyone safe.
Just imagine if VW's emission control software had been open source. People would have looked at it and said "WTF! What are you doing?" Now I know some of you are saying "Ah, but they could publish a good recipe and then bake the bad one". True, but it'd still be a lot easier to catch them as you could bake the recipe they published and then compare it to the pre-baked version. In the VW example the pre-baked version would somehow, mysteriously have way better mileage. And because people know how to cook, they'd know there are only a couple of ingredients that could be fiddled with to achieve that result.
VW is not the only one hiding things in their closed source software. Most programs that you find in "App Stores" are closed source and many of them do their best to take your personal information, often without permission. These activities would be plainly visible if people could look at the source code, as would many vulnerabilities or things like back doors in the program.
This is where we are hopefully heading. Many programs are now open source; many are still secret. We will probably never get to a 100% open source world. But as people learn more about the open source movement, and realize that programming is just a learned skill like cooking, instead of seeing it as a magical "something" that only rare geniuses can do, there will be more and more pressure for companies to open their source code, or for software repositories like "App Stores" to include a way to also download the source code for a program.