Why do some rolling pins only have one handle?
As Megha says above, this kind of rolling pin is rolled by pressing on the pin itself, as you can see in the video.  I have two such rolling pins in different sizes.  The knob is not used when rolling out dough.

The single knob is there for picking up the pin, both by itself, and for transporting rolled pasta, pizza, and tart crust dough.  In the video, you can see Sra Simili rolling up the dough around the pin, and using the pin to move it.  If the dough has been stretched large enough to be the length of the pin, then you use the knob handle so that you can still pick it up.

The knob is also a useful handle when you use the pin to chase children out of the kitchen.