I use Ubuntu for a the following reasons:
1/ a more secure web browsing experience. I don't want to have to worry about viruses, etc. Ubuntu is basically the place I go for all the risky web activity. Think NSFW or what would not be appropriately discussed on this forum. I basically just go around clicking random download links. Never had a virus. (In the off chance that I do have a virus, though, I use ANOTHER Linux distro for more secure activity just to be safe.)
2/ support for more file formats. Windows doesn't support audio formats like .flac or video formats like .flv. I know there are probably some video players to patch up functionality; but, I'm really just not into finding which is right for me.
3/ the Empathy chat client. Before Windows 8, Windows lacked a good IM client. I know there were programs like Pidgin; but, that really bogs down your system. Windows 8 has a built-in messenger app (for Facebook chat and Skype only); but, the problem is that it isn't very reliable with regards to receiving messages. So it is back on Empathy I go.
4/ random open source software. If you're too cheap to go off and buy real software, Linux has you covered with some shoddy alternative programs. Some example of some scarcely used programs I use is a video editor. I don't need to edit videos; but, it doesn't hurt to change the length of a video occasionally.
5/ powerful features like partition editing, bootloader, live media, etc. My computer is ancient and doesn't know how to boot from USB without GRUB. There was this nice GUI bootloader called Plop. Guess what? Complete garbage. I literally have Linux to thank for having Windows 8 on my computer.
6/ it's a nice backup OS. Have you ever worried that your computer is useless because something is going wrong with whatever OS? I never have to worry about that. I always have two backup OSes on my computer.
There are probably more reasons and I could go on and on. Linux simply packs a lot of functionality that Windows doesn't. You just have to be the right person to search for the functionality; but, ultimately, it's a good alternative and I proudly consider Ubuntu to be primary OS.