The real question is, why the hell would you buy a mac. >_<
I usually don't encourage posts like this, but...
I wanted a Unix based system with a nice GUI on top. So I bought a Mac.
In contrast to Windows (which is what I assume you're using): I get
bash instead of
cmd.exe preinstalled, and if I ever do want to use a different OS (Windows, Chromium, Linux) I can just load up a virtual machine. Also, a lot of my favorite software is Mac specific: Textmate, Coda, Colloquy, Xcode, and Schoolhouse, to name a few.
Also, I develop apps for the iOS platform (iPad, iPhone, iTouch...), so I had to own a Mac anyway to run Xcode and the iOS SDK (which wasn't a problem because I was a Mac user before the first iPhone came out).
In contrast to Linux (I've only tried Ubuntu, though): I like
homebrew more than apt-get. I also like the GUI on a Mac more than I like Ubuntu's GUI (which I like more than Windows 7's GUI).