Why do you use a Mac instead of a Windows PC or a Linux Machine?
Pretty simple question. Why did you choose a Mac over a PC or a Linux computer?
Personally, I chose Mac because of the amazing multitasking functionality. Expose/Mission Control is amazing and I haven't seen any 3rd party software on other platforms that work as well. Spaces is great for running different programs in different desktops, so I can have one desktop for schoolwork, one desktop for programming, etc. The full screen app support is also great for single-window applications like FCPX.
And of course what really complements the multitasking is the amazing multitouch trackpad support. On Windows, when I press F11 to make an app fullscreen, only that app can really be open, but on OS X I can use a simple gesture to switch between apps quickly and efficiently. I can use another gesture to get to Mission Control and yet another to access App Expose, and if I want to open another app I can use yet another gesture to open Launchpad. OS X has really nice gestures that flawlessly support up to 5 fingers and I'm still seeing Windows OEMs struggling to make basic 2 finger gestures work.
As a computer scientist I also really like Macs for their amazing keyboards and the Unix based OS. Xcode is also better than 99% of IDEs that I've used in my career.
Of course, there are still a few bugs with OS X. Like when I am using "Show Desktop" and a new window opens up, it appears in the middle of the desktop and not pushed off to the side like all the other windows. Also I wish Launchpad would stutter less and scaled better on larger screens. But OS X combined with amazing Mac hardware is why I choose the Mac.