The button on the top serves only on the Alpa to close the lens to the preset value and release the shutter. I very much like the aperture setting wheel, I find it much more user-friendly than the usual aperture rings. It has big engraved numbers that are easy to read and once you know the lens, you can set the aperture by click stops without even looking at it.
On the bottom of the lens you see the DOF preview button. It makes it easy to focus at full aperture and then close the diaphragm to the preset value in a snap.