Just had to give a shout out to Stallman & GNU. I've seen a lot of mentions of thanks to Linux on here, but Richard will never let us forget that Linux ain't shit w/o GNU software to interact with it.
Just think of the number of GNU programs you've used, just in a typical day on the terminal.
Not everyone writes open source. Let's put the reasons aside, but GPL stuff is unusable outside of open source. MIT and Apache are the licenses that make code really free.
I have to add that GPL licenses would hardly change the intention of creators of software not to publish their source code, instead limiting what libraries they can use and open possibility to sue for a fragments of code that could originate from GPL licensed repositories.
A lot of GNU software has some other FOSS equivalent that it can be replaced with. GCC, however, was basically the only production-worthy FOSS C/C++ compiler for a long time, until Clang came along.
GNU was the very first free Unix reimplementation project. Without it, maybe only excessively expensive commercial Unix systems would be available alongside Windows. Although 386BSD was also an early effort, the intense FUD campaign prevented it from being used for more serious purposes. At the time, GNU/Linux played a crucial role in competing against commercial Unix systems.