Originally, because I was a poor middle school student with a bunch of dumpster hardware. I could not afford a windows license (this was the XP days). I immediately liked Ubuntu (gnome 2 at the time) more than windows, everything felt faster and more customizable. It really screamed on my pentium 3. I used Linux of various flavors all the way through school and continue to use it as my OS of choice to this day. I remember my teachers always being mad that I didn't use "times new Roman" font when I turned in papers, explaining that I used Linux and TNR was not an available font didn't do much for me. I would switch to windows for AAA games back in the day, but that is quickly becoming less necessary.

The biggest benefit I have seen over the years is that it is so much easier to keep old hardware alive (and still secure) with Linux. If your old matching is starting to bog down you can always find a lighter weight distro to load it up with. And when you are ready to upgrade hardware the old stuff can easily be turned into a server, game console, or PC for grandma. Anything to keep it out of a landfill is pretty easy to do. It used to be that you never had to worry about paying for an upgrade either, but now that windows is essentially free for upgrades that is no longer a huge benefit.