Streamlined red, yellow and green stop buttons at the top left of each window are one of the design changes in Apple's OS X 10.10 Yosemite make-over.
The button functions have changed slightly from close, minimise and maximise to close, minimise and full-screen. Along the way, Apple has dropped the full-screen button that sits in the top right of windows.
I rarely use the red, yellow and green buttons to control windows on my MacBook Air. My fingers know the keyboard shortcut for closing a Window.
It's unusual minimise and maximise windows on the 13 inch screen, but the lack of display real estate means I often need to use full-screen mode. Having to use two sets of controls for one set of functions was a minor design flaw, putting it all in one place makes a lot of sense.