I use Linux because it gives me freedom. I can run whatever software I want, and configure my operating system in whatever way pleases me. I get to choose everything from my desktop environment to my file system to my bootloader to my keyboard shortcuts. Apple and Microsoft don't let me choose any of those things.
Because Linux operating systems are so open and understandable, Linux programs (e.g. mpd) can integrate better with the operating system than their equivalents on Windows or Mac. And it's easier to find and install programs for Linux, because Linux distributions come with excellent package managers and repositories. It's as easy to install LibreOffice as any iPhone app.
Another great thing about Linux operating systems is that they provide well-documented interfaces. I can enable or disable a system service using systemd. I can create my own services. I can easily automate tasks such as backups and updates.
I don't think a proprietary system could ever offer so much freedom. Linux is good because individual contributors collaboratively build it up. Without the ability to change and share code freely, this wouldn't be possible.
Although I prefer free software, I'm okay with using proprietary user applications. Sometimes there simply aren't good open source alternatives to the best games, photo/video editing software, accounting/banking/medical software, etc. So it's acceptable that some applications are proprietary--that's what it takes to have niche applications. But I will only use free systems software.