OK, when I was a kid, there was a software application called Appleworks that you could buy for your Mac. It was an office application, so it did word processing and spreadsheets. It saved to its own proprietary format of .cwk
At some point along the way, Apple stopped making Appleworks. Most people saw that coming and converted all their old .cwk documents to something more generic like .doc or even MORE generic (and more open) like .txt or whatever, before getting rid of their old computers and buying a new one on which Appleworks wouldn't run even if only to open their old .cwk documents.
So, what would happen if you were a kid and didn't know any better, and didn't think to change your documents from .cwk to something else? or if you were that kid's parents and weren't really very tech savvy, so you didn't know to convert the documents before Apple killed off the program that created them?
Well, it turns out that you're basically screwed.
Because the .cwk format was closed source, so no application knows how to open them.
You'd think a company would at least have the common decency to say "well, at one point in our history, we created the .cwk format, so our new products, at least, will still support opening .cwk documents" but no, that's not how many companies work.
You might also wish (though you'd be foolish to expect it) that a company might go so far as to say "since we created the .cwk format, we should open up at least the information required to READ .cwk files, so other applications could open those files. ESPECIALLY since we're not even supporting .cwk ourselves any more..."
But again, that's not how many companies work.
In other words, a lot of files out there are now digital paper weights that can not be opened by any modern computer. Did you write the great American novel as a .cwk file? too bad, it's gone. Are important family or financial records in .cwk format? too bad, those are gone too. Are files of pure sentimental value in .cwk files? too bad; your memories are worthless.
Closed source software is truly evil. It doesn't respect you, or your data, or your memories, or art, or self-expression. Oh, maybe it does right now...but just wait. It will screw you over in the end.
That's a depressing way to end this post, so, let's hear about...
Open source software wants you to be able to get to your data FOREVER. This means that its file formats are OPEN so anyone who wants to open them CAN. That doesn't mean Apple or Microsoft will bother letting you do it, but other open source applications will.
The data formats are also based on well-known standards, so even if (and that's a big if, so far; Free Software has been around since about 1983 or so, and there's yet to be a format that's been left behind) - but if a format does go out of fashion and everyone stops supporting it (again, a really unlikely scenario), then someone could still write a program JUST to convert that data to something else. It's basic file parsing; it's done all the time when migrating from one database to another or from one email application to another. Not a big deal - as long as you can look inside the data. And with open source formats, you always can.
So, really, future-proof your digital life. Go open source.